1) Boil your cut up cauliflower in a large pot of salted water for 3 minutes, until tender but still crisp. Pull out your cauliflower, but keep the water to make the pasta in!
2) Melt 2 TBSP. butter in large sauté pan over medium-high heat. Sauté cauliflower for five minutes. Add tomatoes and onions. Cook for a few minutes just to combine all the yummy flavors.
3) In a separate pan, make your Roux by melting 2 TBSP. butter and whisking in 2 TBSP. flour. Cook for a few minutes and then slowly add in your cream and milk. The mixture will thicken pretty quickly.
4) Add your cheese slowly until it is all melted, then sour cream or creme fraiche, 1/2 of your Parm, and mustard. Season with Salt and pepper and remove from heat once it is thick and cheesy and awesome.
5) Cook your pasta in the cauliflower water. Drain the water.
6) Add your cauliflower mixture to the pasta and pour sauce over it. Combine well.
7) Pour into a buttered 13X9 baking dish.
8 ) Top with breadcrumbs and the rest of your Parm.
9) Bake at 350 for 40 minutes until golden brown.