Peel the onion and garlic, bake in a dry hot frying pan (3 minutes). Wash mushrooms. Boil water in a saucepan. Add onion, garlic, dry mushrooms, bay leaf and pepper. Bring to a boil, reduce heat and simmer for 5 minutes. Salt to taste. Strain the broth through a fine sieve and two layers of cheesecloth. Let it cool down a bit.
Preheat the oven to 220º C. Wash the breast, dry it, put it on a baking sheet greased with vegetable oil, sprinkle with oil and cover with a sheet of parchment. Bake 20 min. Cool to room temperature. Remove the meat from the bone, remove the skin. Cut both halves of the breast into thin slices.
Peel the grapefruit. With a thin sharp knife, cut out pieces of pulp.
Pour gelatin with a small amount of warm broth; when it swells, add the rest of the broth; stir thoroughly until the gelatin is completely dissolved. Pour a small amount of broth into a flat dish and let it harden. After that, cut the jelly into small cubes. Place in refrigerator until serving.
Pour a small amount of broth with gelatin into the mold, a layer of 1 cm. Place in the refrigerator so that the broth freezes. Lay the pieces of breast on the frozen broth, alternating them with pieces of grapefruit.
Pour in the remaining broth. Place the mold in the refrigerator to solidify completely. Garnish with prepared jelly cubes.