Use a 4- or 5-quart heavy casserole with a tight-fitting lid. Cook the onions in the lard, covered, over low heat for about 5 minutes. They should become almost pasty, but definitely not browned.
Add chicken and tomato and cook, covered, for 10 minutes.
Stir in paprika. Add ½cup water and the salt. Cook, covered, over very low heat for 30 minutes. In the beginning, the small amount of water will create a steam-cooking action. Toward the end of the 30-minute period, take off lid and let the liquid evaporate. Finally let the chicken cook in its own juices and fat, taking care that it does not burn. (If the chicken is tough, you may have to add a few more tablespoons of water.)
Remove chicken pieces. Mix the sour cream, flour and 1teaspoon cold water, and stir in with the sauce till it is very smooth and of an even color. Add green pepper, replace chicken parts, adjust salt. Put lid back on casserole and over very low heat cook until done.
Just before serving whip in the heavy cream. Serve with egg dumplings.