Arrange the bacon on a rimmed baking sheet and bake at 400°F for 12-20 minutes until crisp. Cool and chop.
Place the chopped red onion in a bowl of cold water for about 10 minutes, then drain well.
In a large bowl, combine the broccoli, drained onion, cranberries, sunflower seeds, and cheddar (if using).
In a small bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, salt, and pepper until smooth. Taste and adjust.
Pour the dressing over the broccoli mixture and toss to coat. Add most of the chopped bacon, reserving some for topping.
Let the salad sit for at least 15 minutes before serving. Scatter the reserved bacon over the top just before serving.