In a skillet over medium heat, melt 1 tablespoon of butter, then add the diced onion, sautéing for about 2-3 minutes until translucent. Add the chopped shrimp and cook until pink and opaque, approximately 5 minutes. Once fully cooked, gently fold in the lump crab meat and remove from heat.

In a medium saucepan over medium-low heat, melt the remaining 2 tablespoons of butter. Whisk in the flour, cooking for 1-2 minutes to create a roux. Gradually pour in 1.5 cups of broth while whisking to prevent lumps. Cook until the sauce thickens, about 5 minutes, then stir in sour cream and softened cream cheese until smooth.

Preheat your oven to 350°F (175°C). Warm flour tortillas briefly to make them pliable. Spoon the seafood filling onto each tortilla, roll tightly, and arrange seam-side down in a greased baking dish.

Pour the creamy sauce evenly over the enchiladas and sprinkle with shredded cheese. Bake for 20-25 minutes, or until the cheese is bubbly and golden.