In a large pot of boiling water, cook noodles according to package directions until just tender.
Drain and transfer to 12 cup casserole dish.
Meanwhile, in a large skillet, heat 2 tbsp of olive oil over medium-high heat.
Saute mushrooms for 5 to 7 minutes or until tender.
Spread over noodles, along with shrimp, scallops, fish medley and clams.
In a small sauce pan, melt remaining butter over medium heat. Whisk in flour, salt and pepper. Cook, stirring constantly for about 2 minutes or until smooth.
Gradually whisk in milk and cook, stirring constantly, until thickened. Pour over noodle mixture and sprinkle with cheese.
Cover and bake in preheated (350F) oven for 40 to 45 minutes or until hot and bubbly.
Remove from oven and preheat broiler. Uncover casserole, sprinkle with paprika and broil for 3 to 5 minutes or until top is golden brown.