Soak chicken in salted ice-cold water for 1 hour. Soak matzo in lukewarm water till soft, then squeeze till dry.
Cook chopped onion, covered, in 2tablespoons of the chicken fat; do not brown. After 5 minutes add chicken livers, parsley and mushrooms. Cook, covered, for another 5 minutes. Let the mixture cool.
Add squeezed matzo and put the whole mixture through the grinder. Add salt and pepper to taste. Add eggs and bread crumbs and mix well. Let the stuffing rest for a couple of hours.
Mix in 6tablespoons chicken fat.
Remove chicken from ice-cold water, wipe it well inside and outside, and salt it. Fill with the stuffing. Sew up the opening.
Spread 2tablespoons of chicken fat on the bottom of a baking pan. Put in tomato, sliced onion and green pepper. Add garlic, a sprinkle of salt and ¼cup water. Spread remaining 2tablespoons fat on top of the chicken, put the chicken in the pan, cover the pan, and cook the chicken on top of the stove over low heat. Baste often.
Preheat oven to 475°F. When the chicken is done, take it out of the pan and keep it warm. Put vegetables through a sieve or blender to make a sauce.
Put chicken back in the pan and place it in the preheated oven for about 10 minutes to make it brown and crisp.
Serve the sauce separately. Accompany with egg noodles, boiled and then sautéed in chicken fat for a few minutes, or rice.