1. Preheat the oven to 375°. Cook the bacon in a 10 1/2-inch cast iron skillet over medium-high heat until crisp. Remove to paper towels to drain; crumble and set aside.
2. Transfer 1 tablespoon of the bacon drippings to a small bowl. Drain off the remaining drippings from the skillet and discard. Wipe the skillet clean, then return the reserved drippings to the skillet. Place in the oven to heat.
3. Beat the eggs in a large bowl. Add the milk, melted butter, cornbread mix, and hot pepper sauce, stirring until blended. Stir in the onion, spinach, shrimp, and 11/2 cups cheese. Carefully remove the skillet from the oven. Pour the batter into the hot skillet. Sprinkle the remaining 1/2 cup cheese over the top.
4. Bake until set and golden brown, 30 to 35 minutes. Remove from the oven and let rest 5 to 10 minutes in the skillet on a wire rack. Sprinkle with the bacon and parsley, if desired; cut into wedges and serve.