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.
Whisk together eggs, milk, chives, salt, and pepper. Stir in sausage and potato. Set aside.
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).
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.
Slice frittata into wedges. Serve warm or at room temperature.
Note: Nutritional analysis is per serving.