Bring a large pot of lightly salted water to a boil. Add linguini pasta, and cook for 6 to 8 minutes, or until al dente. Drain.
Meanwhile, in an electric frying pan, or large skillet saute the red onion and garlic in olive oil. When onion is translucent, add the milk. Cook until bubbles form around the edges of the pan. Add the parsley, chopped green and red bell pepper, broccoli, carrots, mushrooms, shrimp, crab, and scallops and stir until well incorporated.
Remove 1/2 cup of milk from the mixture and place in a small bowl with the flour. Stir until smooth. Add back to skillet with seafood and vegetables. Allow mixture to thicken. Season with salt and pepper to taste.
Pour seafood sauce over drained and cooked linguini noodles. Serve warm.