This is a terrific breakfast or brunch item that can be prepped the night before, so the actual assembly and cooking goes quickly in the morning. A bed of hash brown potatoes forms a crust to which a layer of chopped broccolette is added. A cheesy egg custard then binds everything together and causes the frittata to puff and browns as it cooks. The resulting one-pan meal is light, tender, and full of great flavors. Yummy!
Steam the broccolette until tender, 8 to 10 minutes. Drain immediately and let cool. Chop the broccolette into small (1/2-inch) pieces and set aside. The broccolette can be cooked and chopped, then covered and refrigerated, a day in advance.
Place the eggs in a medium-size bowl and whisk to blend. Add the milk, 1/2 teaspoon of salt, and freshly ground black pepper to taste. Whisk to combine. Stir in the grated cheese and set aside. The custard can be prepared a day in advance of making the frittata, covered and refrigerated.
Position a rack in the middle of the oven and preheat to 375 degrees F.
Place the oil in a 12-inch cast-iron skillet (or other oven-proof, nonstick skillet) and heat over medium heat. Add the onion and cook, stirring constantly, until the onion is lightly golden, about 3 minutes. Add the hash browns, salt them lightly, and cook, stirring frequently, until hot, about 3 minutes. Press the hash browns into an even layer across the bottom of the pan and raise the heat to medium-high. Cook without stirring until the bottom of the potato crust begins to brown, about 5 minutes. Remove the skillet from the heat and spread the broccolette over the potatoes in an even layer. Pour the egg and cheese custard into the skillet.
Bake the frittata until it’s just set (the center may still jiggle a bit), 15 to 18 minutes. Turn the broiler on high and place the skillet about 4 inches below the heat. Cook until the frittata puffs and turns golden brown, 4 to 6 minutes, watching carefully so that it doesn’t burn.
Let the frittata cool for 5 minutes then cut it into wedges to serve.