Preheat the oven to 350ºF. Bring a large pot of water to a boil and salt it generously.
Meanwhile, in a large saucepan, melt butter over medium heat. Add the onion, sprinkle in the salt, and sauté until translucent. Sprinkle the flour, mustard and paprika over the onions, and whisk for 5 minutes making sure the flour is cooked. Gradually whisk in the milk, Simmer, whisking occasionally, for 10 minutes.
Gradually add the cheese to the milk mixture. Taste and adjust seasoning, if necessary, with salt and pepper.
Add the macaroni to the boiling water and cook until just al dente, about 2-3 MINUTES. Drain well, rinse with cool water and then immediately transfer to the cheese sauce. Stir until well mixed, then pour into a casserole dish. Spread the mixture in an even layer. Place the cheese crackers in a food processor and pulse for 10 seconds. Stop and add the melted butter and pulse again for 10 more seconds. (You are looking for a coarse breadcrumb consistency.)
Bake in a baking dish until bubbling and the top is browned, about 30 minutes. Remove from the oven and let rest for 5 minutes. Serve hot.
