Preheat the oven to 400 degrees.

Heat the olive oil and butter in a 12-inch, ovenproof, nonstick skillet over medium-high heat.

Add the potatoes and onion and cook for 10 to 12 minutes, stirring occasionally, until light brown and crispy.

Add the zucchini and red bell pepper and cook for 3 to 4 minutes more, stirring occasionally, 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, Gruyere, salt, and pepper in a bowl.

Pour the egg mixture over the vegetables in the skillet.

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 fresh rosemary.

Bake in the preheated oven for 12 to 15 minutes, uncovered, just until the eggs set.

Remove from the oven and let cool slightly.

Cut the frittata into wedges.

Serve immediately or at room temperature.