Preheat your oven to 375°F (190°C).
In a large bowl, whisk together the eggs, salt, and black pepper until well combined. Set aside.
Heat olive oil in an oven-safe skillet over medium heat. Add the onion, bell pepper, zucchini, and mushrooms. Sauté for about 5 minutes, until the vegetables are slightly softened.
Add the baby spinach to the skillet and cook for an additional 2 minutes until wilted.
Pour the egg mixture over the sautéed vegetables in the skillet. Sprinkle the shredded cheese evenly on top.
Cook the frittata on the stovetop for about 3 minutes, allowing the edges to set.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the frittata is set and golden on top.
Remove from the oven and let it cool for a few minutes. Garnish with fresh herbs if desired.
Slice and serve the frittata either warm or at room temperature. It can be enjoyed on its own or with salsa, kimchi, hot sauce etc…