Cook rice in plenty of water the same way you would cook pasta until al dente. Rice shouldn't be too soft- about 5 minutes. Drain and set aside.
In a frying pan heat olive oil and butter. Cook onions and carrots over low heat for 10 minutes until tender.
In a large bowl combine beef, pork, dill, onions, carrots, rice, salt and pepper.
Roll meatballs, then brown them on all sides in a little oil but don't cook all the way through. Set aside.
In a large and deep pan cook onions and carrots in olive oil and butter over low heat for at least 15 minutes until tender and caramelised, add minced garlic and cook for a few seconds while stirring, add dill, pureed plum tomatoes and stock, turn the heat up and bring to a boil.
Take off the heat and stir in sour cream. Return the meatballs back to the pan and spoon the sauce over them. The sauce should nearly cover them. If it doesn't, add more stock.
Bake covered at 300F/165C for 40 minutes, then uncover and bake 10 minutes longer. (If baked at a higher temperature sour cream tends to separate!)