Cheesy Potato Frittata

Preheat oven to 350°F with rack in upper third. Place potatoes in a large pot and just cover with water; stir in 1 tablespoon salt. Bring to a boil over high. Reduce heat to low and simmer until potatoes are just tender when pierced with a fork, about 3 minutes. Drain; rinse under cold water to stop the cooking. Set aside.

Heat oil and butter in a large cast-iron or oven-safe nonstick skillet over medium until butter foams. Add onion and ½ teaspoon salt. Cook, stirring occasionally, until onion is golden brown, about 15 minutes. Add potatoes and toss to coat. Cook, undisturbed, until potatoes are golden in spots, about 3 minutes. Gently stir; cook for 3 minutes more.

Meanwhile, whisk eggs, cream, bell peppers, black pepper, half the cheese, and remaining 2 teaspoons salt in a large bowl until well combined.

Slowly add egg mixture to potato mixture in skillet and cook, stirring gently and smoothing around edges with a rubber spatula, until eggs are scrambled in spots, about 2 minutes. Transfer to oven and bake until center is just set, 16 to 18 minutes. Remove from oven and increase temperature to broil. Top frittata with remaining cheese. Return to oven and broil until cheese is bubbling and golden, 2 to 4 minutes. Let cool for 5 minutes before slicing. Frittata can be made up to one day ahead and stored once cooled, covered, in the refrigerator. Bring to room temperature and reheat, if desired.