Heat the oven to 325° F.
Grease an 8x8" (or any 2 quart) baking dish.
Mix together the eggs, milk, salt, pepper, garlic powder and red pepper flakes (if using). Set aside.
Add the drained sun-dried tomatoes onto a double layer of paper towels and pat dry, then roughly chop. Stir the tomatoes and sliced scallions into the egg mixture.
Pile the thawed frozen spinach in the middle of a flour sack towel (or any thin towel), cheesecloth or paper towels. Discard any big stems. Wrap up the spinach like a sack and squeeze as much water out as possible. Just when you think you've squeezed out enough, try squeezing out some more.
Stir the spinach into the egg mixture, breaking up any clumps so the spinach is evenly distributed through the eggs. Pour the mixture into the prepared dish. If the onions or tomatoes are clumping together, use a fork to drag the pieces around the dish to distribute them evenly.
Bake for 35-40 minutes, or until the top of the eggs look set and a knife inserted in the middle comes out clean.
Let rest for 5 minutes so the eggs can set, then slice and serve.