1. Preheat oven to 450°. In a 10-in. ovenproof nonstick frying pan or well-seasoned cast-iron skillet, cook bacon over medium-high heat until crisp, about 5 minutes. Transfer bacon with a slotted spoon to paper towels; reserve 2 tbsp. drippings in pan (discard the rest) and set aside.
2. Whisk together eggs, milk, chives, salt, and pepper. Stir in bacon and artichoke hearts. Set aside.
3. Return skillet to medium-high heat. Add leeks and sauté in reserved bacon drippings until softened, 1 to 2 minutes. Reduce heat to medium. Add egg mixture and stir to combine. Cook 1 minute, then gently stir in 1/2 cup fontina and the parmesan. Cook without stirring until edges of frittata are set and the center is still a bit soft, about 2 minutes (edges should appear firm when pan is gently shaken; the top layer should appear wet).
4. Sprinkle remaining 1/4 cup fontina over top and transfer skillet to preheated oven. Bake until eggs are fully set and the top is light golden brown, 5 to 7 minutes. If top has not browned, broil frittata about 5 in. from heating element, keeping oven door ajar, until lightly golden, about 1 minute.
5. Slice frittata into wedges. Serve warm or at room temperature.
Note: Nutritional analysis is per serving.