Preheat oven to 400°F. In a large ovenproof, nonstick skillet, cook bacon over medium-high heat, stirring often, until crisp, about 10 minutes. Drain on paper towels.
Pour off fat from skillet. Spread spinach in bottom of skillet.
In a large bowl, combine eggs, milk and cheese; season with salt and pepper. Blend well and pour over spinach. Scatter bacon over mixture. Cook over medium heat, lifting sides of frittata with a spatula to let uncooked egg flow underneath, until lightly browned around edges, about 5 minutes.
Transfer skillet to oven and bake, uncovered, until puffed and lightly browned on top, about 10 minutes. Gently shake pan to loosen. Cut frittata into wedges and serve with a salad and some bread.