Asparagus Quiche

1 bag frozen hash browns
1 1/2 teaspoons kosher salt, divided
3/4 teaspoon freshly ground black pepper, divided
2 tablespoons vegetable oil
3 tablespoons unsalted butter, divided
1 onion, chopped
6 large eggs, room temperature
1 1/4 cups half-and-half
1 teaspoon mustard powder
Pinch of grated nutmeg
1 tablespoon finely chopped tarragon
1 cup Fontina or Brie cheese, grated
1 cup cheddar cheese, grated
1/2 bunch asparagus (about 1/2 pound), ends trimmed

Preheat oven to 350°F.
Toss hash browns with 1 tsp. salt and 1/2 tsp. pepper in a large bowl.
Heat oil and 2 Tbsp. butter in a 10" cast-iron skillet over medium-high until butter is melted.
Add potatoes and immediately start forming into a crust by pushing potatoes flat against bottom and sides of pan with a 1/2-cup dry measuring cup.
Continue cooking, pressing potatoes up sides of pan if they start shrinking,
until potatoes are bound together and bottom of crust is starting to brown, about 10 minutes. Remove pan from heat and set aside.
Meanwhile, melt remaining 1 Tbsp. butter in a small skillet over medium heat.
Add onion and sauté until translucent, 5-6 minutes; set aside.
Whisk eggs, half-and-half, mustard powder, nutmeg, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper in another large bowl.
Whisk in tarragon and set aside.
Sprinkle cheese, and sautéed onion evenly over bottom of crust, then pour in egg mixture.
Arrange asparagus decoratively on top. Bake until quiche is set and crust is well browned, 30-35 minutes.
Let cool to room temperature before cutting into wedges and serving from the pan.