Cut a thin slice off the top of each tomato. Scoop out pulp, leaving a 1/2-in. shell. Invert tomatoes on paper towels to drain. Chop pulp, reserving 1 cup.
In a skillet, saute mushrooms in 2 tablespoons butter until tender. Combine egg yolks and sour cream; add to mushrooms. Stir in 1/4 cup bread crumbs, salt, thyme, pepper and reserved tomato pulp. Simmer, uncovered, until thickened, about 1 minute. Spoon about 1/3 cupful into each tomato.
Place in an ungreased 11x7-in. baking dish. Melt remaining butter; toss with remaining bread crumbs. Sprinkle over tomatoes. Bake, uncovered, at 350° for 30-35 minutes or until heated through.