Frittatas are wonderfully adaptable. Slices of this family-sized omelet can be served hot or cold, for breakfast, lunch or dinner. Hemingway put frittatas in between slabs of country-style bread for hearty sandwiches. This flavorful version might be best if it goes right in your mouth.
Place oven rack in the middle of the oven and preheat the broiler.
Cook the bacon in a large skillet over medium heat until crisp, about 8 minutes.
Transfer to paper towels, reserving rendered bacon fat in the skillet.
In the same skillet cook the onion and garlic until softened, 5 minutes.
Transfer the onion mixture to a bowl.
Cook the spinach in the skillet until it is wilted and tender, about 2 minutes.
Work in batches if necessary.
Transfer the cooked spinach to a colander and squeeze out as much liquid as possible.
Chop the spinach and add it to the onion and garlic mixture.
Melt the butter in a large, ovenproof skillet over medium-high heat.
In a bowl, whisk the eggs, milk, salt, and pepper together and pour into the skillet.
Gently stir in the spinach, onion, bacon, and minced chives.
Cook until the eggs are almost set, about 5 minutes, occasionally lifting up the edges of the frittata and tilting the pan so the uncooked egg mixture flows underneath.
Sprinkle cheese over the top of the eggs and place the skillet in the oven.
Broil until the frittata is set and the cheese begins to brown, about 3 minutes.
Cut the frittata into wedges and serve immediately with sour cream and hot sauce alongside.