How to Make It
Arrange racks in upper and lower thirds of oven; preheat oven to 400°F.
Place beets and turnips on a baking sheet lined with parchment paper. Drizzle with 2 tablespoons oil, and sprinkle with 3/4 teaspoon salt and 1/2 teaspoon pepper; toss gently to coat. Arrange in a single layer on baking sheet.
Place Gorgonzola in a large bowl; mash with a fork until smooth. Add 6 tablespoons oil; stir well with fork. Stir in melted butter. Add bread pieces; toss gently to coat. Arrange bread in a single layer on a baking sheet lined with parchment paper.
Place bread on upper oven rack and beet mixture on lower oven rack. Bake in preheated oven 15 minutes. Stir vegetables and bread, and return both to oven on same racks. Bake until bread is crisp and browned, 8 to 10 minutes. Remove bread from oven, and set aside. Continue to roast vegetables until tender when pierced with a knife, about 10 minutes. Cool to room temperature, about 30 minutes.
Meanwhile, stir together vinegar, 1/2 cup water, sugar, and 1 1/2 teaspoons salt in a small saucepan; bring to a boil. Add onion; boil 1 minute. Remove from heat, and let cool to room temperature, about 30 minutes. Drain onions, reserving 3 tablespoons pickling liquid.
Whisk together reserved pickling liquid, mustard, honey, remaining 3/4 teaspoon salt, and remaining 1/2 teaspoon pepper in a large bowl. Gradually add remaining 1/4 cup oil, whisking constantly, until emulsified. Add kale, beets, and turnips; toss gently to combine. Transfer to a large, shallow bowl. Top with pickled onions and croutons.