In a 10-inch ovenproof skillet, heat the olive oil. Add the onion, garlic, sage and crushed red pepper and cook over moderately high heat, stirring, until the onion is softened and lightly browned, about 3 minutes. Add the tomatoes and their juices, season with salt and pepper and bring to a simmer. Stir in the bread and sprinkle with the feta. Pour the egg mixture on top and bake for 20 minutes, until golden brown. Let cool slightly, then serve.