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.
Preheat oven to 325 degrees F. Coat a 12-cup muffin tin with cooking spray.
Bake until firm to the touch, about 25 minutes. Let stand 5 minutes before removing from the tin.
Heat oil in a large skillet over medium heat. Add finely diced potatoes, diced onion and 1/4 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 1/2 teaspoon salt in a large bowl. Stir in chopped spinach and the potato mixture. Divide the quiche mixture among the prepared muffin cups.
Percentages are based on a diet of 2000 calories a day.