Step 1: Preheat your oven to 350°F (175°C) with a rack in the middle position.
Step 2: In a large, oven-safe, non-stick skillet (preferably 10-inch), heat the olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes until softened and translucent.
Step 3: Add the minced garlic and bell peppers to the skillet. Continue to cook for another 5-7 minutes, stirring occasionally, until the peppers are tender-crisp.
Step 4: Stir in the spinach. Cook for 2-3 minutes, until it has completely wilted and any excess moisture has evaporated. If there's too much liquid, drain it off.
Step 5: In a large bowl, whisk together the eggs, milk/cream (if using), salt, black pepper, and half of the shredded cheese until well combined and slightly frothy.
Step 6: Distribute the cooked vegetables evenly in the bottom of the skillet. Pour the egg mixture over the vegetables. Gently stir to ensure the vegetables are well distributed.
Step 7: Sprinkle the remaining half of the shredded cheese and the cherry tomato halves (cut side up) over the top of the egg mixture in the skillet.
Step 8: Transfer the skillet to the preheated oven. Bake for 20-25 minutes, or until the frittata is set in the center and lightly golden brown on top. A knife inserted into the center should come out clean.
Step 9: Carefully remove the skillet from the oven. Let the frittata cool in the skillet for 5-10 minutes before slicing.
Step 10: Slide the frittata onto a cutting board, slice into wedges, and serve warm or at room temperature. Enjoy!