1. Melt butter in large saucepan. Add mushrooms. Sauté until golden and soft. Reduce heat to low. Add spinach and milk. Stir to incorporate. Cook until warm throughout. Remove from heat; let cool to room temperature. 
2. Preheat oven to 350° F. Prepare 12-cup muffin pan by spraying with nonstick cooking spray and lining pan with rounds of parchment paper cut to size (or use muffin liners). In a large bowl, beat eggs with a whisk to incorporate some air. Stir in the mushroom and spinach mixture. Add cheese and salt. Evenly divide mixture into muffin pan. Bake for 15 to 20 minutes or until firm to the touch. Remove pan from oven. Let cool to room temperature. If not using muffin liners, run a spatula around edges, and turn pan over to remove frittatas. Individually wrap in plastic wrap; refrigerate or freeze. If frozen, thaw in fridge overnight before reheating and serving. Remove plastic wrap, and reheat in microwave for 30 to 40 seconds. 

Calories: 78, Fat:6 g (3 g Saturated Fat), Cholesterol:93 mg, Sodium: 144 mg, Carbohydrates: 2g, Fiber: 1 g, Protein: 5 g.