I feel like every Swedish family has a recipe of their own that is passed down and gently modified by each generation. Here is a link to a good “baseline” which aligns very well with my family one if you remove the sugar [1]. It is in Swedish though, but translation engines tend to do a good job with “simple” text like recipes.

[1]: https://www.ica.se/recept/klassiska-kottbullar-712807 (Swedish)

Now, I prefer to have them with boiled potatoes (some prefer mashed), lingonberries (not lingonberry jam though, just boiled gently with a tiny touch of sugar and leave the berries intact if you can), pickles (we have a family recipe for that as well), and of course creamy “brunsås” gravy. Here is a baseline recipe for the gravy [2], I have never used butter myself though and make sure to make it after the meatballs so that you can use whatever flavour you have left in the frying pan as additional stock for the sauce (this is not mentioned in the recipe – despite it being gravy – and should also remove the need to add salt and pepper). I also prefer to make the gravy thick and sticky, so that it will stick to the boiled potatoes if you want to go down that route.

[2]: https://www.ica.se/recept/brunsas-720341 (Swedish)

Happy cooking!