From the Art of Jewish Cooking by Jennie Grossinger:
***Fillings For Kreplach***
Heat the fat (I use oil) in a skillet and cook the meat and onions in it for 10 minutes, stirring frequently. Add the salt and pepper. Cool before placing in squares of dough.
*Kasha: 1 cup minced onions 3 tablespoons chicken fat or butter (depending on whether you are serving a meat or dairy dish) 1 1/2 cups cooked kasha 1/4 teaspoon pepper
Lightly brown the onion in the fat or butter. Stir in the kasha and pepper.
*Cheese-Potato: 1/2 cup minced onions 3 tablespoons butter 1 1/2 cups mashed potatoes 1/2 cup pot cheese 1 teaspoon salt 1/8 teaspoon pepper 1 egg Sour cream
Lightly brown the onions in the butter. Add the potatoes, cheese, salt, pepper and egg, beating until smooth. Serve with sour cream.
*Chicken: 1 1/2 cups ground cooked chicken 4 tablespoons browned minced onions 1 egg yolk 3/4 teaspoon salt 1 Dash pepper 1 tablespoon minced parsley
Blend all the ingredients together.
*Chicken liver: 1/2 pound chicken livers 1/2 cup minced onions 2 tablespoons chicken fat 2 hard-cooked egg yolks 1 teaspoon salt 1/8 teaspoon pepper
Cook the livers and onions in the fat for 10 minutes, mixing frequently. Grind or chop the livers, onions, eggs, salt and pepper. Cool before placing in squares of dough.