Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Prepare the Enchilada Filling: In a medium bowl, whisk together the 6 large eggs and 1/4 cup milk. Season with salt and pepper. In a non-stick skillet over medium heat, melt the 1 tablespoon butter. Pour in the egg mixture and scramble until just set but still moist. Remove from heat.
Step 3: In a separate bowl, combine the scrambled eggs, 1 cup shredded cheddar cheese, 1/2 cup shredded Monterey Jack cheese, and the optional 1/2 cup cooked breakfast potatoes or hashbrowns. Stir in the 2 sliced green onions.
Step 4: Assemble the Enchiladas: Lay out a flour tortilla. Spoon about 1/2 cup of the egg mixture down the center. Roll the tortilla up tightly and place it seam-side down in the prepared baking dish. Repeat with the remaining tortillas and filling.
Step 5: Make the Sausage Gravy: In the same skillet (or a separate one if preferred), cook the 1/2 pound pork breakfast sausage over medium heat, breaking it up with a spoon, until browned and cooked through. Drain off most of the grease, leaving about 2 tablespoons in the skillet with the sausage.
Step 6: Sprinkle the 2 tablespoons all-purpose flour over the cooked sausage and stir well. Cook for 1-2 minutes, stirring constantly, to cook out the raw flour taste. Gradually whisk in the 1 1/2 cups whole milk, ensuring no lumps form. Bring the mixture to a simmer, stirring continuously, until the gravy thickens. Season with salt, black pepper, and the optional 1/4 teaspoon garlic powder to taste.
Step 7: Pour the warm sausage gravy evenly over the assembled enchiladas in the baking dish.
Step 8: Bake for 20-25 minutes, or until the enchiladas are heated through and the gravy is bubbly. Serve immediately and enjoy!