Preheat your oven to 350°F (175°C).
Grease the muffin tin with olive oil to prevent sticking.
In a mixing bowl, whisk together the eggs, milk, salt, and black pepper until well combined.
Add the chopped spinach, diced bell peppers, halved cherry tomatoes, diced red onion, and sliced mushrooms to the egg mixture. Stir until the vegetables are evenly distributed.
Pour the egg and vegetable mixture evenly into the prepared muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 20-25 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 gently running a knife around the edges and removing them from the muffin tin.
Serve warm or let cool completely and store in an airtight container in the fridge for a quick breakfast option throughout the week.