Wash clams under cold water. Throw away any cracked or open clams. Keep clams soaking in cold water until ready to use.
In a large pan (one that comes with a lid), heat the butter over medium high heat. Add the diced onion, and garlic. Saute for a few minutes, until the onion has softened but not turned brown. Add the diced mushroom and saute for another few minutes. Add the tomato, salt, pepper, basil and paprika. Cook for another few minutes, then remove the mixture from the pan and keep aside in a small bowl.
In the same pan, add your shrimp in a single layer. Salt and pepper the shrimp, and cook for 1-2 minutes until pink and opaque on the bottom, then flip over, salt and pepper this side, and cook for another 1-2 minutes until cooked through. Remove shrimp to a separate plate.
Increase heat to high, and using the same pan, drain the clams and add them to the pan. Immediately add the white wine or grape juice and close the pan with a tight fitting lid. Allow clams to steam 7-8 minutes, then remove the pan from the heat and throw away any clams which have not opened up. Remove the other clams and keep them aside, leaving the liquid in the pan.
Let the liquid in the pan settle for a few minutes, then using a fine mesh strainer, strain the broth into a measuring cup (you are using a strainer to get rid of any sand or grain). Wash the pan to get rid of any sand, then put it back on the heat, turning it down to medium. Add the broth back into the clean pan.
Add the butter and flour, and whisk until mixture thickens, about 5 minutes. Add the milk, and the cooked mushroom tomato mixture back to pan and whisk to combine. Sauce won't be very thick.
Add the cooked pasta to the sauce and toss to combine. Add the cooked clams and prawns and stir lightly to incorporate into the pasta, keeping most on the top of the pasta. Top with chopped parsley to garnish.