01 - Set the oven temperature to 375°F.
02 - Heat olive oil in a 10–12 inch ovenproof skillet over medium heat. Add sliced red onion and diced zucchini; cook for 3 minutes until they soften slightly.
03 - Stir in chopped spinach and halved cherry tomatoes, cooking for an additional 2 minutes until the spinach wilts.
04 - In a mixing bowl, whisk together eggs, heavy cream, salt, and black pepper. Fold in crumbled feta, grated Parmesan, chopped basil, and chives.
05 - Pour the egg mixture evenly over the sautéed vegetables in the skillet and gently stir to distribute.
06 - Cook on medium heat for 3 to 4 minutes until the edges begin to set.
07 - Transfer the skillet to the preheated oven and bake for 12 to 15 minutes, until the frittata is puffed and the center is fully set.
08 - Allow the dish to cool slightly. Slice and serve warm or at room temperature.