Bring a large pot of water to a boil. Add salt to season the cooking water, then add the pasta. Cook for 5 minutes, then add the cauliflower and cook for 3 minutes more or until pasta is cooked al dente and the florets are just tender. Drain well and return to the pot.
While the pasta cooks, heat a medium sauce pot over medium-low heat. Add the extra-virgin olive oil and heat with the butter until it melts. Add the onions and cook for 3 to 5 minutes to sweat them out and turn the juices sweet. Raise the heat a bit, then whisk in the flour, cayenne, and paprika. Whisk together until the roux bubbles up, then cook for 1 minute more. Whisk in the milk and stock and raise the heat a bit higher to bring the sauce to a quick boil. Once it bubbles, drop the heat back to a simmer and cook until the sauce thickens, 3 to 5 minutes.
Add the cheese to the thickened sauce and stir to melt it, a minute or so. Stir in the mustard and season the sauce with salt and pepper. Pour over the cauliflower and cooked pasta and toss to combine. Adjust the seasonings, transfer to a large platter, and serve.
Recipe courtesy of Rachael Ray