Preheat oven to 350 degrees and spray a 9x13 baking pan with grease.
To create cheese sauce: melt the butter in a saucepan over medium heat. Add in the green/red pepper and sauté until tender. Sprinkle in the oregano, garlic salt, pepper, cayenne, and flour. Stir to combine and allow to cook for about 1 minute. Whisk in the milk and once slightly thickened, add in the cheese. Once cheese has melted, stir in the sour cream and remove from heat. Set cheese sauce aside.
To prepare shrimp filling: melt the butter in a skillet over medium heat. Sauté onion until tender. Add the shrimp and cook about 3 minutes. Add the tomatoes and cook an additional minute.
To assemble: fill each tortilla with the shrimp mixture. Roll the tortilla tightly up and place seam down in prepared pan. Once each tortilla has been filled and mixture used up, pour the cheese sauce evenly on top of pan ensuring you cover all enchiladas. Sprinkle 1/2 cup cheese on top and place in oven.
Bake for about 30 minutes, or until cheese is melted and pan bubbles.
Remove and enjoy!