1. Preheat the oven to 400 degrees.

2. Heat the olive oil and butter in a 12-inch, ovenproof, nonstick skillet over medium-high heat. Add the potatoes and onion and cook 10 to 12 minutes, stirring occasionally, until light brown and crispy.

3. Add the zucchini and red bell pepper and stir and cook 3 to 4 minutes more, until the zucchini is lightly cooked but still crunchy. Add the spinach and stir and cook about 1 minute longer, until the spinach wilts. Whisk together the eggs, Gruye?re, salt, and pepper in a bowl and pour over the vegetables. Remove the skillet from the heat and gently shake the skillet to distribute the eggs evenly through the mixture. Divide the cream cheese into 1/4 -inch pieces and distribute evenly on top of the egg mixture. Sprinkle with rosemary. Bake 12 to 15 minutes, uncovered, just until the eggs set. Remove from the oven, cut into wedges, and serve immediately, or let cool and serve at room temperature. Frittata can be made up to 2 hours in advance.

Variations: Like omelets, frittatas can be flavored with an almost unlimited list of ingredients, from herbs to cheeses to smoked meats and more. Check the refrigerator for leftovers, see what’s fresh in the garden, and then decide what to add to your frittata.