01 - Preheat oven to 350°F (175°C). Grease a standard 12-cup muffin tin lightly or line with paper liners.
02 - Heat a splash of oil in a skillet over medium. Sauté onion until soft and translucent, about 3 to 4 minutes. Add chopped spinach and cook until wilted or warmed if using thawed spinach. Remove from heat and allow to cool slightly.
03 - In a large mixing bowl, whisk eggs, ricotta cheese, milk, salt, pepper, and nutmeg until thoroughly combined and smooth.
04 - Fold shredded cheddar, grated Parmesan, sautéed vegetables, and fresh herbs if using into the egg mixture until evenly distributed.
05 - Spoon the mixture evenly into muffin cups, filling each approximately three-quarters full.
06 - Bake for 18 to 22 minutes until the tops are lightly golden and muffins are set and firm to touch.
07 - Let cool in pan for 5 minutes. Transfer muffins onto a wire rack to cool further. Serve warm or at room temperature.