01 - Preheat oven to 375°F and generously grease a 6-cup muffin tin with cooking spray or oil.
02 - Heat vegetable oil in a small skillet over medium flame. Add sliced scallions and minced garlic, sautéing 1 to 2 minutes until aromatic. Incorporate spinach and cook an additional minute until wilted, then remove from heat and allow mixture to cool slightly.
03 - In a mixing bowl, whisk together eggs, milk, soy sauce, toasted sesame oil, black pepper, and salt. Blend in cheese, if using.
04 - Fold cooled scallion-garlic-spinach mixture into the seasoned eggs, ensuring even distribution.
05 - Fill each greased muffin cup about three-quarters full with the mixture, distributing evenly among all cups.
06 - Transfer tin to the oven and bake for 16 to 20 minutes, or until centers are puffed and fully set.
07 - Cool frittatas for 2 to 3 minutes, then gently run a knife around edges and remove from tin. Serve warm or at room temperature.