Preheat oven to 375°.
Melt 2 tablespoons butter in a skillet over medium heat. Add onion and garlic and cook until softened. Pour into a large mixing bowl and set aside.
In the same skillet, melt 1/4 cup butter, add flour and whisk, cooking 1-2 minutes. Add diced jalapeno and cumin, whisking to combine. Cook for another minute.
Whisk in sour cream, milk and salsa verde. Cook, whisking until smooth and thickened. Remove from heat and set aside.
Peel, devein and chop the shrimp.
Add chopped shrimp, crab, cilantro and 2 cups of shredded cheese to the bowl of onions and garlic. Mix well.
Spoon filling into flour tortillas and roll tightly.
Place enchiladas seam side down in a large baking dish. Pour sauce over the top and sprinkle with remaining cheese.
Bake at 375° for 30- 35 minutes until bubbly and lightly golden.
Top with pico de gallo before serving.