Preheat the oven to 350°F (175°C). Grease a muffin tin with olive oil or non-stick cooking spray.
In a large bowl, whisk together the eggs, milk, garlic powder, salt, and pepper.
Add the shredded cheddar cheese (if using) and stir until combined.
Stir in the diced bell peppers, onions, zucchini, spinach, and sliced mushrooms. Mix until evenly distributed.
Pour the egg mixture evenly into the muffin cups, filling each cup about 3/4 full.
Bake in the preheated oven for 18-22 minutes or until the frittata cups are set and lightly golden on top.
Remove from the oven and let cool for a few minutes before serving.
Garnish with fresh herbs, such as parsley or chives, for added flavor and color.
Serve warm or at room temperature.