Shock the onions in an ice water bath for 10 minutes to temper the sharp flavor, and pat dry.
Place the bacon, broccoli, cauliflower and onions in a large salad bowl. In a small bowl, whisk the sugar, vinegar and Dijon mustard. Slowly drizzle in the oil, whisking to make an emulsion. Mix in the mayonnaise and seafood seasoning. Season with salt and pepper. Pour the dressing over the vegetables and toss.