How to Make It
Bring vinegar, 11/2 tsp. salt, sugar, and 3/4 cup water to a boil in a medium saucepan over high. Cook, stirring occasionally, until sugar is dissolved, about 2 minutes. Place onions in a heat-safe glass measuring cup. Pour hot vinegar mixture over onions, pressing onions down into mixture to be sure they are fully submerged. Let mixture cool to room temperature, about 2 hours. Transfer mixture to the refrigerator and chill, covered, overnight or up to 2 weeks.
Place a rimmed baking sheet in oven. Preheat oven to 450°F with baking sheet in oven. Toss broccoli with 1 Tbsp. olive oil and 1/2 tsp. salt in a medium bowl. When oven is up to temperature, place broccoli in a single layer on hot baking sheet. Roast broccoli in preheated oven 12 minutes, or until tender and slightly browned. Let cool completely, about 20 minutes. Whisk together mayo, honey, dijon, remaining tablespoon olive oil, remaining 1/4 teaspoon salt, and 2 tablespoons pickled onion liquid in a small bowl. Toss broccoli with apple, fennel, fennel fronds, pecans, bacon, 1/2 cup pickled onions, and dressing.