1. Preheat oven to 400°.
2. Heat olive oil and butter in a medium (9- or 10-inch) ovenproof skillet over medium heat. Add potatoes, and cook, stirring occasionally, about 6 minutes or until potatoes are slightly tender and brown. Add onions, and sauté about 1 minute or until slightly wilted and translucent.
3. Add spinach and arugula, and cook, stirring constantly, about 1 minute or until spinach wilts.
4. Whisk together eggs and next 4 ingredients in a bowl, and pour over vegetables.
5. Remove skillet from heat; drop ricotta by the teaspoon onto frittata, and sprinkle with basil.
6. Bake at 400°, uncovered, for 13 to 15 minutes or just until eggs are set around the edges and slightly loose in the center. Remove skillet from oven, and let stand for about 5 minutes, loosely covered. Cut into wedges, and serve immediately, or cool and serve at room temperature.