Rinse tomatoes; cut 1/2 inch horizontally off the top of each and save. With a spoon, scoop soft pulp and seeds from tomatoes, leaving a 1/4- to 1/3-inch-thick wall; save pulp mixture. Drain tomatoes, cut side down, on towels.
Chop enough pulp mixture to make 2/3 cup; reserve remainder for other uses. In a bowl, mix pulp, turkey, shallots, garlic, egg, basil, parsley, marjoram, oregano, bread crumbs, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Fill tomatoes equally with the seasoned turkey mixture; set cut side up in a 2- to 3-quart shallow casserole. Set tomato tops, cut side down, between tomatoes.
Bake in a 375° oven until filling is firm when gently pressed in center, 35 to 40 minutes (30 to 35 minutes in a convection oven). Set tomato tops on filling. Add salt and pepper to taste. Garnish with herb sprigs.