Preheat your oven to 180°C (350°F). Lightly oil the muffin tin or line it with paper liners for easy removal.
In a sauté pan, lightly cook the diced bell peppers and chopped spinach until just tender and fragrant; set aside to cool slightly.
In a large mixing bowl, whisk together the eggs, milk or cream, salt, and pepper until the mixture is smooth and slightly frothy.
Fold the sautéed vegetables, cooked meat, shredded cheese, and chopped herbs into the egg mixture, ensuring everything is evenly distributed.
Divide the filling mixture evenly into each muffin cup, about 2-3 tablespoons per cup, filling them about three-quarters full.
Gently pour the remaining egg mixture over each filled cup, making sure not to overfill and leaving a little space for expansion.
Sprinkle a little extra cheese and herbs on top if desired, then place the muffin tin in the oven.
Bake for 20-25 minutes, until the muffins are puffed, golden, and set in the center when tested with a toothpick.
Remove the muffin tin from the oven and let the muffins rest for 5 minutes; this helps them set and makes them easier to remove.
Gently loosen the edges with a spatula or knife, then lift the muffins out of the tin and serve warm or at room temperature.
