Cook pasta according to package instructions adding 1 tablespoon of salt to the water for added flavor. Add the cauliflower to the pot 2 minutes before the pasta is done cooking. Alternatively, you can steam the cauliflower in a separate pot until fork tender.
While the pasta is cooking, begin making the cheese sauce. Head the butter in a medium sauce pot over medium heat until melted. Once melted, sprinkle in the flour. Whisk to incorporate, about 30 seconds, before pouring in the milk. Increase heat to high and continue whisking. Once it comes to a slow boil and bubbles form around the edge, reduce heat to low and simmer, whisking constantly, until white sauce starts to thicken, about 2-3 minutes.