Prepare the dressing: In a medium bowl, whisk together the mayonnaise, apple cider vinegar, sugar, Dijon mustard, salt, and pepper until smooth and well combined. Set aside.

Prepare the vegetables: Ensure the broccoli florets are washed and dried thoroughly. In a large mixing bowl, combine the broccoli florets, chopped red onion, raisins, and sunflower seeds (if using).

Combine the salad: Pour the prepared dressing over the broccoli mixture. Toss gently until all ingredients are evenly coated.

Add bacon and chill: Gently fold in the crumbled bacon, if using. Cover the bowl tightly with plastic wrap.

Refrigerate for at least 30 minutes (or up to 4 hours) to allow the flavors to meld and slightly soften the broccoli.

Taste and serve: Before serving, give the salad one final toss. Adjust seasoning (adding more salt or sugar if necessary). Serve cold.