In an 8-inch ovenproof skillet, heat the oil. Add the shallots and cook over moderate heat until softened, about 7 minutes. Add the tomato and prosciutto and cook, stirring, until warm, about 2 minutes. Pour in the beaten eggs and cook until beginning to set on the bottom, about 3 minutes. Poke the mozzarella cubes in the eggs. Bake until just firm when lightly pressed, about 20 minutes.