Bring a large pot of salted water to a boil and prepare an ice bath.
Blanch the broccolini in boiling water for 2 minutes until tender-crisp.
Transfer broccolini to ice bath for 1 minute, then drain and pat thoroughly dry.
In a large mixing bowl, whisk together mayonnaise, apple cider vinegar, and sugar.
Add the broccolini, red onion, dried cranberries, sunflower seeds, and bacon to the bowl.
Toss the mixture until all components are evenly coated in the dressing.
Season with salt and black pepper to taste.
Refrigerate for at least 30 minutes to allow flavors to meld before serving.