Preheat your oven to 400°F. Generously butter a 9-inch glass or ceramic pie dish.
Frozen broccoli: Place the chopped broccoli in a large microwave-safe bowl. Add ¼ cup of water. Cover and microwave on high for 6 minutes, stirring after the first 3 minutes. Drain well.
Fresh broccoli: Place the broccoli florets in a large microwave-safe bowl. Add ¼ cup of water. Cover and microwave on high for 3 minutes. Drain well, then chop the florets into small pieces.
In a large bowl, whisk the eggs, sour cream, kosher salt, black pepper, and garlic powder.
Stir in the broccoli, scallions, and cheese.
Transfer the mixture to the prepared pie dish. Bake the quiche until it's golden brown, and a knife inserted in its center comes out clean, about 30 minutes.
Allow the quiche to cool and set in the pan on a wire rack for about 15 minutes, then slice it into 8 triangles and serve.
Video
Notes
You can double the recipe, bake it in a 9 X 13-inch baking dish, and cut the quiche into squares.
Butter works best for greasing a pie plate. I prefer it to a cooking spray or brushing the pan with oil.
To ensure the leftovers keep well and do not get soggy, let the quiche cool and slice it. Then, let the slices cool for 30 more minutes on a cooling rack. Store the leftovers on a layer of paper towels in an airtight container in the fridge and replace the paper towels daily. Stored this way, the leftovers keep for about 4 days. Once completely cool, you can also freeze individual slices in freezer bags.