1. Preheat your oven to 375°F (190°C). Grease a muffin tin with olive oil or use muffin liners.
2. In a skillet over medium heat, add olive oil and sauté onions and bell peppers until soft, about 5 minutes. Add the chopped spinach and cook until wilted. Remove from heat and let cool slightly.
3. In a large bowl, beat the eggs and milk together until well combined. Stir in cheese, salt, pepper, and garlic powder.
4. Add the cooled vegetable mixture to the egg mixture, stirring until evenly distributed.
5. Pour the mixture into the prepared muffin tin, filling each cup about 3/4 full. This ensures they have room to puff up as they bake.
6. Bake for 20-25 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
7. Let them cool in the pan for a few minutes before transferring to a wire rack. Serve warm or let them cool completely before storing.

Remember, every oven is different, so keep an eye on them during the last few minutes. They should be puffed and slightly golden.