Preheat the oven to 450. In a large bowl, whisk the eggs and yogurt together. In an oven-safe skillet, heat the oil and add the onion, garlic, sage, tarragon and crushed red pepper flakes. Cook over medium heat, until onion has softened, 3 to 5 minutes. Add the tomatoes and their juice, salt and pepper and bring to a simmer. Stir in the bread and feta. Pour the egg mixture on top and bake for 20 minutes, until golden brown. Let cool and serve.