Begin by washing and chopping all vegetables into uniform pieces to ensure even cooking. Pat spinach dry after washing to prevent excess moisture.
Heat olive oil in a 10-inch oven-safe skillet over medium heat. Add the sliced onions and cook until translucent, about 3-4 minutes.
Add the bell peppers and continue cooking for another 3 minutes until slightly softened. Add the garlic and cook for 30 seconds until fragrant.
Add the zucchini to the skillet and cook for 2-3 minutes until slightly softened but still maintaining some structure.
Fold in the spinach and cook just until wilted, about 1-2 minutes. Season the vegetable mixture with salt, pepper, and optional red pepper flakes.
In a large bowl, whisk the eggs with milk until well combined and slightly frothy. Pour the egg mixture over the vegetables in the skillet, gently tilting to ensure even distribution.
Sprinkle the crumbled feta and fresh herbs evenly across the top.
Let the frittata cook undisturbed on the stovetop for 3-4 minutes until the edges begin to set. Meanwhile, preheat your broiler.
Transfer the skillet to the oven and broil for 4-5 minutes until the top is golden and the eggs are completely set. The center should be just firm to the touch - a knife inserted in the middle should come out clean.
Allow the frittata to cool for 5 minutes before slicing and serving.