Preheat the oven to 375°F and grease all 12 cups of a standard muffin tin with the olive oil.
In a large bowl, whisk the eggs, milk, salt and black pepper until smooth.
Stir in the spinach, feta, bell pepper and onion until evenly combined.
Divide the mixture among the 12 muffin cups, filling each about two-thirds full.
Bake for 18 to 20 minutes, until the centers are set and the edges are lightly golden.
Cool in the pan for 5 minutes, then run a knife around the edges and lift the muffins out.
Serve warm, or refrigerate up to 4 days and freeze up to 2 months; reheat in the microwave for 30 to 45 seconds.