Preheat your oven to 400 degrees F. Generously butter a 9-inch pie dish.
Place the frozen vegetables 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.
In a large bowl, whisk together the eggs, sour cream, kosher salt, black pepper, garlic powder, and onion powder. Stir in the thawed veggies and shredded cheese.
Transfer the mixture to the prepared pie dish. Bake until golden brown and a knife inserted in the 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, before slicing and serving.
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Notes
Don't serve this quiche right away - as tempting as that might be. Allow it to cool and set for about 15 minutes before slicing and serving. Its texture and flavor will improve after this short rest.