Assume you need 2,500 calories/day. Suppose you need a third of those at lunch, that's 830. (And given the traditional light continental breakfast, I'd expect an even larger lunch to compensate, so probably closer to 1,000.)
A single sandwich with three layers of bread and three single layers of cheese or meat, can't be over 400 calories.
I'd be starving and unproductive all afternoon. What's the secret?
You'd think that, but it's a tricky thing listening to our bodies: That feeling you associate with hunger can also be triggered by your stomach being empty or being on the far side of a sugar/carb spike.
Real hunger is a very different feeling. I often fast(†) 2-3 days in a row (sometimes as many as 4) and I definitely don't feel real hunger the first two days of not eating (and rarely before day four). It feels awful in a way you're not likely to confuse with the feeling you have midday, but fortunately dealing with an empty stomach is easy: Have some water (or a coffee/tea; decaf is ok). 20 minutes later your stomach has something in it and something to do (more if it's cold water) and you can get on with your day.
The second part is that sugar-spike: That's a killer for me. If you're getting too many of your calories from sugar your body will respond fast to the lack of sugar (even though you've got plenty of calories) and you will feel hungry even if you're not. Everyone who has eaten a candy-bar on an empty stomach has felt this a few hours later, but there's not much you can do except eat less sugar, and get your calories from something less spiky like fat and protein. This is probably the single biggest reason low-carb diets are successful, because you don't get that carb craving feeling.
(†) I usually mean zero-calories by this, but I travel around twenty times a year and it's often really helpful to get some new calories at the right-time-of-day to help adjust to the local timezone, so I might have as much as 600kcal in a day and still consider it "fasting". This is still far less than what many people have in even a single meal, and I'd have no problem with my energy-level or really anything else doing it continuously except for the fact that I really do like to eat.
One trick is to simply eat food that digests a bit more slowly. I like a breakfast of yoghurt and musli with some fruit mixed in. The musli keeps me going quite long and I find I don't get hungry quite as soon as when eating some pastry or other junk that people eat for breakfast. For lunch, I try to keep it light. Social pressure to go for big lunches with colleagues is making this hard. But I often go for salads or simple sandwiches. I'll eat something proper early evening when I have time to enjoy it.
Back in the day when I was regularly visiting colleagues in the US, I noticed that most of my US colleagues were quite a bit heavier than most of my European colleagues. Not a surprise because getting a healthy lunch was super hard. The service industry seems to specialize in trying to kill you with food. Most people were going for what I would classify as junk food. Every day. Even a sandwich was more like a heart attack on a plate. The service industry there is biased towards stuffing people with way too much food so they tip more. So, a sandwich would contain about 3X the recommended daily intake of meat; about the same amount of cheese and be dripping with fatty mayonaise and all wrapped in what passes for bread in the US. Typically comes with completely redundant crisps as well.
A quick check of Wikipedia's handy breakfast-per-country list [1] gives:
80% of Norwegians eat breakfast (frokost) daily, mostly at home. The most common breakfast is open sandwiches (smørbrød), often whole wheat bread, with cheese, often Jarlsberg, Norvegia or brunost, cold cuts,[71] leverpostei, jam etc. Common drinks are water, various types of coffee such as French press, cafe latte or espresso, milk and juice. Another common breakfast is breakfast cereals like corn flakes eaten with milk, kulturmelk, or yogurt. Whole-grain porridges with regular milk or butter are also eaten by some. More ample breakfasts may include fish, a diverse array of cheese, eggs, bacon, breads, and hot and cold cereals eaten in various combinations.[67] Pastries such as croissants or pain au chocolat have become increasingly common since the 1990s.
So I'd have 100% whole grain bread with lean meat and some cheese. I'd use a flavorful cheese so I didn't have to use too much in order to not go over budget (in terms of calories). I used things like mustard instead of butter.
With this I could go about 5 hours without feeling particularly hungry (office job).
Well, your first mistake is there
So enough with the snark, please.
2,500 cal/day is well-known as the average male guideline, while 2,000 cal/day is the average female guideline.
None of this changes my original point. Obviously nobody's healthily surviving on 1,200 calories/day.
People generally don’t count calories, certainly not how many percentage per meal they need. There’s also breakfast, snacking, dinner, more snacks etc.
The snark started with you insinuating that somehow a time-tested diet is not possible, because it's different from yours.