Cook rice in salt water according to package instructions. Meanwhile peel and finely dice the onion. Heat oil in a pot. Fry the minced onion for about 4 minutes until crumbly. Fry onion for about 1 minute.
Stir in tomato paste and sweat briefly. Add tomatoes, 150 ml water and stock. Bring to the boil and season with salt and pepper. Cover and simmer for about 5 minutes.
Add frozen vegetables and cook for another 8-10 minutes. If necessary, drain the rice and put it into the mincing pot. Season to taste with salt and pepper.