In a large skillet, melt 1 tablespoon of butter over medium heat. Sauté the diced onion until it turns translucent, about 2-3 minutes, then add the peeled and chopped shrimp. Cook for an additional 3-5 minutes until the shrimp is pink and opaque. Gently fold in the lump crab meat and set the mixture aside to cool slightly before assembling your Seafood Enchiladas.

In a medium saucepan, melt the remaining 2 tablespoons of butter over low heat. Once melted, whisk in the flour, stirring constantly for about 1 minute until lightly golden. Gradually add 1.5 cups of broth, whisking continuously until the mixture is smooth and thick, approximately 3-4 minutes. Remove from heat and stir in the sour cream and cream cheese until fully blended and creamy.

Preheat your oven to 350°F (175°C). Warm the flour tortillas in a microwave or on a skillet for a few seconds to make them pliable. Place a generous spoonful of the shrimp and crab filling in the center of each tortilla, roll them tightly, and arrange them seam-side down in a greased 9x13 inch baking dish, ensuring no filling spills out.

Pour the luscious creamy sauce evenly over the assembled seafood enchiladas, ensuring they are well-covered. Sprinkle the shredded Monterey Jack cheese generously on top for that extra melty goodness. Bake in the preheated oven for 25-30 minutes, or until the cheese is bubbly and golden, and the edges of the enchiladas start to crisp slightly.

Once baked, remove the Seafood Enchiladas from the oven, allowing them to cool for a few minutes. This will enhance their flavors and make serving easier. Garnish with fresh cilantro or avocado slices, and enjoy this warm and comforting dish with your favorite sides for a delightful family feast!