In a 12-inch ovenproof nonstick skillet, heat the oil. Add the egg-vegetable mixture and cook over moderately low heat, stirring occasionally, until starting to set at the edge, 3 minutes. Dot the ricotta on top and garnish with the zucchini blossoms, if using. Transfer the skillet to the oven and bake for 20 to 25 minutes, until the frittata is just set. Let cool slightly, transfer to a platter, cut into wedges and serve.