1. Preheat oven to 350°F. Coat a 12-cup nonstick muffin tin with cooking spray.
2. In a large bowl, whisk eggs with salt and pepper until blended.
3. Warm oil in a small skillet over medium-high heat. Sauté onion until softened but not browned, 3 to 5 minutes. Add spinach and sauté 1 minute longer. Remove from heat and cool slightly.
4. Whisk onion mixture into eggs. Divide among muffin cups, top each with feta and bake until set in center, 25 to 30 minutes.
Try it another way:
Ham and Swiss: Use 10 eggs. Divid 3/4 cup diced lean ham and 1/2 cup shredded Swiss cheese among cups before adding eggs. Bake as directed.
Fresh Herb: Whisk 12 eggs with 1/2 tsp. salt and 1/4 tsp. pepper. Sauté a small chopped onion in 3 Tbsp. olive oil until softened. Add a minced garlic clove; sauté 1 minute. Whisk into eggs with 2 Tbsp. chopped fresh herbs (chives, parsley or thyme). Divide among cups; bake as directed.