Preheat oven to 325°F. Coat a 12-cup muffin tin with cooking spray.

Heat oil in a large skillet over medium heat. Add potatoes, onion and ¼ teaspoon salt and cook, stirring, until the potatoes are just cooked through, about 5 minutes. Remove from heat and let cool 5 minutes.

Whisk eggs, cheese, milk, pepper and the remaining ½ teaspoon salt in a large bowl. Stir in spinach and the potato mixture. Divide the quiche mixture among the prepared muffin cups.

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

To make ahead: Individually wrap in plastic and refrigerate for up to 3 days or freeze for up to 1 month. To reheat, remove plastic, wrap in a paper towel and microwave on High for 30 to 60 seconds.