In a small bowl whisk together mayonnaise, sugar, and vinegar. Set aside.
In a large bowl combine cauliflower, broccoli, cheese, onion, bacon, and sunflower seeds.
Pour dressing over the top of the vegetables and stir until evenly coated. Serve within 30 minutes to 1 hour after adding dressing for best results. Refrigerate leftovers for up to 4 days.