01 - Set oven to 350°F and lightly grease a standard 12-cup muffin tin or line each cup with paper liners.
02 - Heat a skillet over medium and sauté the diced onion in a small amount of oil until soft and translucent, about 3 to 4 minutes. Add the spinach and cook until wilted or just warmed if using thawed spinach. Remove from heat to cool slightly.
03 - In a large mixing bowl, whisk eggs, ricotta cheese, milk, salt, black pepper, and nutmeg until the mixture is fully blended and smooth.
04 - Fold in shredded cheddar cheese, grated Parmesan, the sautéed vegetables, and fresh herbs if using. Stir gently until evenly distributed.
05 - Evenly divide the combined mixture among the muffin cups, filling each approximately three-quarters full.
06 - Place tin in the oven and bake for 18 to 22 minutes until the muffins are set and the tops are lightly golden.
07 - Allow to cool in the pan for 5 minutes, then transfer muffins to a wire rack. Serve warm or at room temperature as desired.