Preheat oven to 350°F. Coat a 12-cup nonstick muffin tin with cooking spray.
In a large bowl, whisk eggs with salt and pepper until blended.
Warm oil in a small skillet over medium-high heat. Sauté onion until softened but not browned, 3 to 5 minutes. Add garlic and sauté 1 minute longer. Remove from heat and cool slightly.
Whisk onion mixture and herbs into eggs. Divide among the 12 muffin cups and bake until set in center, 25 to 30 minutes.