1. Preheat oven to 400ºF.
2. In a large nonstick, ovenproof skillet, heat oil over medium until it shimmers. Add pancetta. Sauté for 6 minutes or until crisp and browned. Drain on paper towels.
3. Add onion and potatoes to skillet and cook for 8 minutes or until softened. Add garlic and cook for 1 minute more.
4. Whisk eggs, milk, salt, and pepper. Stir in cheese and chives. Pour eggs into skillet and stir in pancetta. Cook over medium for 3 to 5 minutes, shaking pan occasionally and lifting edges of frittata with a spatula so uncooked egg runs underneath. Remove from heat when eggs are halfway set.
5. Transfer skillet to oven. Bake for 8 to 10 minutes or until frittata is puffed and golden. Let sit for 5 minutes. Cut into wedges.