Preheat the oven to 350 degrees and spray a 9 x 13-inch casserole dish with nonstick cooking spray.
Defrost the spinach in the microwave or on the stovetop, put the thawed spinach in a fine meshed strainer, and press out the excess water, using the back of a spoon, bottom of a bowl or your hands.
In a large bowl, beat the eggs and stir in the remaining ingredients, including the spinach.
Pour the mixture into the dish and bake it, uncovered, for 40 – 50 minutes until it starts to brown on the edges and a knife inserted in the center comes out clean. (Alternatively you can refrigerate it for up to 12 hours before baking it, although it might need about 5 more minutes if baking it straight from the refrigerator.) Serve it immediately, topped with salsa, if desired, refrigerate it for up to 3 days, or freeze it for up to 3 months.