Grilled Sweet Potatoes

Name: Grilled Sweet Potatoes
Ingredients: 3 Tbsp. vegetable oil, divided, plus more for grill; 3 large sweet potatoes (about 2 lb. total), scrubbed, sliced into ¼-thick rounds; 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt, divided; 1 tsp. freshly ground pepper, divided; 1 bunch scallions, trimmed; 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, plus leaves for serving; Flaky sea salt