Preheat oven to 400. Heat olive oil and butter in 9 or 10 inch oven proof skillet over medium high heat. Add onion and sauté for 2 minutes, onion will slightly brown. Add peppers, zucchini, stir and sauté for 2-3 min. Add diced tomatoes, stir and cook for an additional minute. Meanwhile, in large bowl whisk together eggs, milk, spinach and salt. Fold in swiss cheese. Pour over veggies. Remove skillet from heat. Drop teaspoons of ricotta onto frittata. Top with tomato slices and sprinkle with spinach. Bake uncovered for 13-15 min or until eggs are set around edges and slightly firm in center. Remove skillet from ovena dn let stand for 5 minutes. Cut into wedges and serve.
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