1. Spray two 13×9-inch freezer-safe baking dishes or pans with cooking spray; set aside.
2. In a large bowl, toss cheese with flour; set aside.
3. For sauce, in a medium saucepan, melt ¼ cup butter over medium heat. Whisk together milk and cornstarch, then stir into butter. Bring to a gentle boil over medium-high heat; reduce heat.
4. Cook and stir over medium-low heat for 2 minutes or until thickened. Add 2 cups cheese mixture, yogurt, and 2 minced jalapeños. Cook and stir for 1 minute; remove from heat and set aside.
5. For filling, in a large nonstick skillet, melt remaining ¼ cup butter over medium heat. Add onion, garlic, and remaining 2 minced jalapeños. Cook for 3 to 4 minutes or until onion is softened, stirring occasionally.
6. Add half each of the shrimp and fish and cook for 3 to 5 minutes or just until shrimp are opaque; transfer to a large bowl.
7. Add remaining shrimp and fish to skillet and cook; then add to first batch of seafood.
8. Toss with ½ cup cilantro, chili powder, and cumin. Stir in 1 cup sauce.
9. To assemble, spoon about ⅓ cup of the filling along the center of each tortilla; roll up then place seam side down in prepared baking dishes, arranging filled tortillas in two crosswise rows in each dish. Divide sauce in half and spoon over filled tortillas in each dish. Divide remaining cheese between the two dishes.
10. Serve Now: Preheat oven to 375°F. Bake, uncovered, for 20 to 30 minutes or until heated through and sauce is bubbly around edges. Let stand 5 minutes before serving. Garnish with additional cilantro and tomatoes, if desired.
11. Serve Later: Completely cool assembled enchiladas in baking dish. Cover baking dish with foil; freeze up to 1 month. Thaw overnight in refrigerator. Preheat oven to 375°F. Bake, covered, for 10 minutes. Uncover and bake 20 to 25 minutes more or until bubbly around edges. Garnish as directed for Serve Now.
12. * Note: Chile peppers contain volatile oils that can burn skin and eyes. When working with jalapeños, wear protective gloves. 

Each serving contains: 544 calories, 27 g fat, 16 g saturated fat, 0 g trans fat, 157 mg cholesterol, 786 mg sodium, 37 g carbohydrates, 1 g fiber, 5 g sugar, 38 g protein. Daily values: 18% vitamin A, 14% vitamin C, 48% calcium, 12% iron.