Marinated Beets with Potatoes and Horseradish

• 3 medium beets (about 1 pound)
• 7 tablespoons olive oil, divided
• 2 tablespoons Sherry vinegar or red wine vinegar, divided

Kosher salt, freshly ground pepper

• 1 pound waxy potatoes (such as Yukon Gold), cut into 1-inch pieces
• 3 scallions, thinly sliced
• 1 cup coarsely chopped mustard greens
• ¼ cup chopped fresh dill
• 2 tablespoons chopped fresh parsley
• 2 tablespoons freshly grated horseradish or 1 prepared horseradish
• 1 tablespoon fresh lemon juice
• ⅓ cup crème fraîche

1. Preheat oven to 400°. Wrap beets in foil and roast directly on rack until tender, about 1 hour. Let cool, peel, and cut into ¾" pieces. Place in a small bowl and toss with 2 Tbsp. oil and 1 Tbsp. vinegar; season with salt and pepper. Let sit at room temperature 30 minutes.
2. Meanwhile, toss potatoes on a large rimmed baking sheet with 1 Tbsp. oil; season with salt and pepper. Roast, tossing occasionally, until golden brown and tender, 25–30 minutes. Let cool slightly. Transfer potatoes to a large bowl and add scallions, mustard greens, dill, parsley, horseradish, lemon juice, remaining 4 Tbsp. oil, and remaining 1 Tbsp. vinegar. Toss to combine; season with salt and pepper. Add beets and toss again. Serve drizzled with crème fraîche.
3. Do Ahead: Beets can be marinated 3 days ahead. Cover and chill.