Step 1

Preheat oven to 325 degrees F. Liberally coat a 12-cup muffin tin with cooking spray.

Step 2

Heat oil in a large skillet over medium heat. Add onion and 1/8 teaspoon salt; cook, stirring, until starting to soften, about 3 minutes. Add bell pepper and oregano; cook, stirring, until the vegetables are tender and starting to brown, 4 to 5 minutes more. Remove from heat and let cool for 5 minutes.

Step 3

Whisk eggs, feta, milk, pepper and the remaining 1/8 teaspoon salt in a large bowl. Stir in spinach, olives and the vegetable mixture. Divide among the prepared muffin cups.

Step 4

Bake until firm to the touch, about 25 minutes. Let stand for 5 minutes before removing from the tin.