Notes: Cooktop temperatures and ovens can vary greatly, so it is always important to follow our visual cues, especially when cooking eggs. You can use any type of cooked sausage (pork or chicken) for this frittata; we liked the andouille, Cajun-style, and sun-dried tomato varieties.
1. Preheat oven to 450°. In a 10-in. ovenproof nonstick frying pan or well-seasoned cast-iron skillet, heat olive oil over medium heat, add sausage and saute until lightly browned, 7 to 10 minutes. Transfer sausage with a slotted spoon to paper towels; reserving oil in pan.
2. Whisk together eggs, milk, chives, salt, and pepper. Stir in sausage and potato. Set aside.
3. Bring skillet to medium-high heat. Add leeks and sauté in reserved oil 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.