Heat the olive oil in a saucepan, sauté the onion cubes, dust with flour, roast briefly, deglaze with the tomatoes (add water for the soup, without adding water it makes a simple tomato sauce) simmer for 30 minutes, add salt, pepper and possibly a sauce. Herbs, season to taste.
Rice, noodles (leftovers from the day before) or baked peas are suitable as soups.