Bring a large sauce pot of water to a boil. Salt the water, add the pasta and cook to al dente.
Meanwhile, in another sauce pot, melt the butter over medium heat, add the garlic and onions and saute until translucent and fragrant. Stir in the flour and cook for 1 minute more. Whisk the milk into the sauce and heat until thickened. Season with the paprika, dry mustard and black pepper, to taste. Melt in the cheese. Drain the pasta, and add it to the sauce along with the pimientos. Serve immediately or transfer it to a shallow dish and brown under the hot broiler for a crunchy top.