Grilled Sweet Potatoes

Prepare a grill for low heat; oil grate with vegetable oil. Toss 3 large sweet potatoes (about 2 lb. total), scrubbed, sliced into ¼"-thick rounds, with 2 Tbsp. vegetable oil, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ½ tsp. freshly ground pepper in a medium bowl to coat. Place 1 bunch scallions, trimmed, on a rimmed baking sheet and drizzle with remaining 1 Tbsp. vegetable oil; turn to coat.

Arrange sweet potatoes and scallions on opposite sides of grill. Grill sweet potatoes until charred in spots and tender, 10–12 minutes per side; transfer to a platter. Grill scallions, turning halfway through, until tops are charred and whites are tender, about 5 minutes.

Transfer scallions to a food processor; pulse until finely chopped but not puréed. Add 1 garlic clove, finely chopped, ½ cup crème fraîche, ½ cup mayonnaise, 2 Tbsp. fresh lemon juice, 1 Tbsp. prepared horseradish, ¼ cup coarsely chopped parsley, and remaining 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and ½ tsp. freshly ground pepper. Pulse until well combined.

Drizzle sauce over sweet potatoes. Top with parsley leaves; sprinkle with flaky sea salt.