Preheat oven to 350 degrees. Spray an 8x8" baking pan with nonstick spray.
Place cauliflower in a large microwave-safe bowl; cover and microwave for 3 minutes. Uncover and stir. Re-cover and microwave for 2-3 minutes, until hot. Drain excess liquid. Roughly chop cauliflower, return to the bowl, and cover to keep warm.
In a large pot, cook pasta per package instructions, about 8 minutes. Drain pasta and stir into cauliflower. Cover to keep warm.
In a medium microwave-safe bowl, combine cheese wedges, shredded cheese, milk, sour cream and mustard. Mix well. Microwave for 1 minute, or until cheeses have melted. Whisk until smooth.
Add cheese mixture to the pasta and cauliflower. Stir to coat. Transfer contents to prepared baking pan.
In a medium microwave-safe bowl, microwave butter for 20 seconds, or until melted. Add panko breadcrumbs and seasonings, and stir to coat. Evenly distribute mixture over the contents of the baking pan.
Bake 15-20 minutes, until topping is golden brown.