Not many vegetables and definitely not many fruits, although on a tour we did pass through an isolated little town in the Gobi that our driver said was known for its vegetables. As far as I could tell, they only seemed to have a few large gardens. Which I think proves the rule. But as you've pointed out there are plenty of other resources that discuss that, likely more accurate than my anecdotes.
Mongolia is pretty large. IIRC the south is mostly desert, the east more grassy plains, and the west very mountainous. I'm sure the cuisine varies. Nonetheless, AFAIU steppe cultures have very similar diets--heavy meat consumption, particularly sheep, goat, camel, etc.
My wife and I went in 2012 for about a week. Most of that time was spent with expat family working in Ulaanbaatar, where we took a few excursions to parks and monuments not too far from the city.
We took a 3-day trip to the Gobi Desert on a very typical itinerary where you hire a driver and translator and stay with a few host families. Because of tourism and the mining industry, the Gobi families were settling down more. Our host families, while still remote and dispersed, lived in Gurs that hadn't been moved in years. (Whereas traditionally you moved roughly once a year, cycling through grazing areas.) At the time travel books recommended the Southeast for a more "traditional" experience. But you really shouldn't miss the Gobi if you're doing a family trip or a short tour.
I'm hardly particularly knowledgable about the country or culture. But what was most striking was how the land was open and accessible--so very little private property, with the legal right for natives and (IIUC) visitors to roam and camp where ever they please so long as they're not disturbing anyone else.[1] It's basically a dream for anyone who loves the outdoors. I'd hardly classify myself as a world traveler, nor an avid outdoorsman. But I have hiked and camped rural Ecuador and visited rural parts of Borneo. The sense of openness, freedom, remoteness, and safety in Mongolia just seemed incomparable to anywhere else. But who knows how long that will last.
[1] There's an imaginary boundary (if not a fence) around a Gur or collection of Gurs that demarcates de facto private from public land. And Mongolians traditionally keep a native breed of guard dog for protection, so you're encouraged to get vaccinated for rabies. But outside the cities families are so dispersed (on the order of miles) that you're unlikely to accidentally intrude upon anyone's space.