In a heavy skillet over medium heat, melt the butter and saute the onions and garlic until soft. Add the flour and stir for about 2 minutes.
Add the milk and cook over medium heat, stirring constantly, about 5 minutes. Add the cheese and stir until the cheese melts.
Preheat oven to 400 degrees F.
Season the fish with salt, pepper, cumin, paprika, onion powder and garlic powder. Place the fish in lightly greased individual baking dishes. Top with the shrimp and add the sauce on top.
Sprinkle each with parmesan cheese. Place in the oven and bake for about 8 to 10 minutes or until bubbly. Remove from oven and serve warm with lime wedges on the side.