• Preheat oven to 375°F. Spray a 9×13-inch baking dish with nonstick cooking spray and set aside.

Pasta

• Cook the pasta in a large pot of salted boiling water according to package directions until just al dente. Do not overcook; it will finish baking in the oven. Drain and set aside.

Seafood Base

• In a large skillet over medium heat, melt butter. Add the diced onion and cook for 3 to 5 minutes until softened. Add the minced garlic and cook for 1 minute.
• Stir in the shrimp, imitation crab, chili powder, cumin, and black pepper. Cook for 2 to 3 minutes until shrimp are just starting to turn pink. They should be slightly undercooked, as they will finish cooking in the oven.
• Remove from heat and stir in 1 cup shredded Monterey Jack cheese and cilantro. Set aside.

Creamy Sauce

• In a medium saucepan over medium heat, melt the 4 tablespoons butter. Whisk in the flour and cook for 1 to 2 minutes to form a roux.
• Slowly whisk in the milk, sour cream, jalapeño, salt, pepper, and cumin. Cook, stirring constantly, until the sauce thickens slightly, 3 to 5 minutes.
• Remove from heat and stir in the 1 cup shredded Monterey Jack cheese until melted and smooth. This makes about 1 ¾ cups sauce.

Assembly

• In a very large bowl (or directly in the skillet if large enough), combine the cooked pasta, seafood mixture, and most of the creamy sauce, reserving ½ cup for the top. Toss gently to coat everything evenly.
• Transfer the mixture to the prepared 9×13-inch baking dish. Pour the reserved ½ cup sauce over the top, spreading it into an even layer. Then, sprinkle with the remaining ½ cup shredded Monterey Jack cheese.
• Bake uncovered for 20 to 25 minutes, or until bubbly, the top is golden, and the shrimp are fully cooked.
• Remove from oven and let rest 5 minutes. Top with chopped cilantro, diced tomatoes, and dollops of sour cream before serving.