In a large skillet, melt a tablespoon of butter over medium heat. Once melted, add the diced onion and sauté for 3–4 minutes until soft and fragrant. Next, toss in the peeled and chopped shrimp, cooking for an additional 3–5 minutes until they turn opaque. Gently fold in the lump crab meat and season with salt and pepper to taste, then set aside.

In a medium saucepan, melt 3 tablespoons of butter over medium heat. Whisk in 3 tablespoons of flour for 1–2 minutes until lightly golden. Gradually pour in 1.5 cups of broth, stirring continuously until the mixture thickens and bubbles, about 5 minutes.

Remove the saucepan from heat and stir in 0.5 cup of sour cream and 4 oz of softened cream cheese. Blend well until the sauce is creamy and smooth, ensuring there are no lumps. Adjust consistency by adding extra broth if necessary.

Preheat your oven to 350°F (175°C). Warm the tortillas in a dry skillet to prevent cracking when rolled. Spoon the seafood filling onto each tortilla, roll them tightly, and place them seam-side down in a greased baking dish. Save some seafood filling for later.

Pour the creamy sauce evenly over the rolled enchiladas in the baking dish. Sprinkle 2 cups of shredded Monterey Jack cheese on top, ensuring the enchiladas are covered.

Transfer the baking dish to the preheated oven and bake for 20–25 minutes until the cheese melts into a gooey, golden layer.

Once baked, remove the seafood enchiladas from the oven and let them rest for about 5 minutes. Garnish with fresh cilantro or avocado slices, if desired, and serve warm.