In a medium bowl, whisk together the mayonnaise, sour cream, sugar, and vinegar until smooth and well combined. Add the Dijon mustard and stir until incorporated. Set aside at room temperature while you prepare the other components. This dressing will emulsify better if it's not ice cold when mixed with the pasta and vegetables.

Bring a large pot of salted water to a boil and cook the pasta according to package directions until al dente. While the pasta cooks, cut the broccoli into small 1/2-inch florets, finely chop the red onion, and shred the cheddar cheese. Once the pasta is done, drain it thoroughly and rinse with cold water, stirring occasionally, until completely cooled—this stops the cooking process and prevents the pasta from becoming mushy in the final salad.

Cook the bacon until crispy, then crumble it into bite-sized pieces and set aside. In a large mixing bowl, combine the cooled broccoli florets, chopped red onion, and shredded cheddar cheese. Season with salt and black pepper, tossing gently to distribute the seasonings evenly. I like to add the cheese at this stage rather than mixing it directly into the dressing, so it doesn't start melting too early and get clumpy.

Add the cooled pasta from Step 2 to the broccoli mixture in the bowl, then pour the dressing from Step 1 over everything. Toss gently but thoroughly until all components are evenly coated. Fold in the crumbled bacon, being careful not to crush it. Refrigerate for at least 30 minutes before serving to allow the flavors to meld and the salad to chill completely. I find the salad tastes even better if you let it sit for a couple of hours—the broccoli softens slightly and absorbs more of the creamy dressing.