Soak the bread in the milk, in a large bowl for about 30 minutes, or until it has absorbed all the milk and is very soft. Add the meat, egg, onion, and allspice, and season with salt and pepper. Knead together well with your hands, then form into small balls about the size of walnuts, rolling them between your palms so that they are compact and won't fall apart when cooking.
Heat 2 1/2 tablespoons of the butter with olive oil in a nonstick skillet. Fry the meatballs in batches, turning them once during cooking. (You will have to work quite quickly and take care not to burn the butter and oil. If neccessary, wipe out the skillet between batches and start again with a little less butter and oil.) Transfer the cooked meatballs to a heavy-bottomed saucepan with any onion that is on the bottom of the skillet and continue cooking the next batch.
Sprinkle the flour into the skillet and mix with a wooden spoon until it is smooth. Add the remaining butter and let it melt. Continue cooking, stirring almost continuously, until it is a golden color. Remove the pan from the heat and very slowly pour in 2 cups of hot water, standing back a bit. Mix in quickly, then return the pan to the heat. Stir in the sour cream and mix well, then carefully pour over meatballs. Season lightly with salt and pepper and cook, covered, over a very low heat for 10-15 minutes, until you have a thick, creamy sauce with soft meatballs to serve with berry jam.