Prepare the Filling: Heat a skillet over medium heat. Add a tablespoon of butter and diced onion, sautéing until the onion is soft and translucent, about 3-4 minutes. Add peeled and chopped shrimp, cooking until they turn pink and opaque, roughly 4-5 minutes. Stir in lump crab meat, season lightly with salt and pepper, then remove from heat.

Make the Creamy Sauce: In a medium saucepan, melt 2 tablespoons of butter over medium heat. Whisk in 2 tablespoons of all-purpose flour and cook for about 1 minute until it turns a light golden color. Gradually add 2 cups of broth while stirring continuously, allowing the mixture to thicken for about 3-5 minutes. Remove from heat before stirring in ½ cup sour cream and 4 oz cream cheese until smooth and creamy.

Assemble the Enchiladas: Warm 8 flour tortillas in a dry skillet or microwave for about 20 seconds to make them pliable. Spoon a generous amount of seafood filling into the center of each tortilla. Roll them tightly and place them seam-side down in a greased baking dish, leaving space between each enchilada.

Top and Bake: Preheat oven to 350°F (175°C). Pour the creamy sauce over the assembled enchiladas, covering them evenly. Sprinkle 1 cup of shredded Monterey Jack cheese on top, bake for 20-25 minutes until golden and bubbly.