Preheat oven to 425°. Grease a 9" springform pan with cooking spray and set on a rimmed baking sheet.

Place frozen hash browns in a large heatproof bowl and microwave on high until almost all liquid has evaporated, 2 to 3 minutes.

To bowl with hash browns, add cheddar, butter, garlic powder, salt, and pepper and mix until well combined.

Transfer hash brown mixture to prepared pan. Evenly press across bottom and up sides of pan to about ½" from top of pan, making sure to cover any holes or cracks.

Bake crust until edges are starting to brown, about 40 minutes.

Preheat oven to 375°. In a large bowl, whisk cornstarch and 1 egg until smooth and no lumps remain. Add sour cream, milk, mustard, salt, pepper, and remaining 5 eggs and whisk until smooth.

In a large skillet over medium-high heat, cook bacon, stirring occasionally, until just starting to brown and crisp, 6 to 8 minutes. Add white scallion parts and half of green scallion parts and cook, stirring, until bacon is crispy and scallions are softened and brown, 1 to 2 minutes more. Transfer to a paper towel-lined plate.

Sprinkle half of cheddar across base of crust. Top with bacon mixture and remaining green scallion parts. Slowly pour egg mixture into crust. Top with remaining cheddar.

Bake quiche until just set and there’s a slight jiggle in the center when pan is shaken, 40 to 50 minutes. Let cool 30 minutes.

Run a knife around edges of pan before unmolding. Serve quiche warm or at room temperature.