2 pounds baby Yukon Gold potatoes, halved 1 cup chopped yellow onion (from 1 medium onion) 1/2 cup chopped celery (from 2 medium stalks) 2 tablespoons olive oil 3 medium garlic cloves, chopped (1 Tbsp.) 2 (1-lb.) smoked ham hocks 4 cups lower-sodium chicken stock 1/2 cup heavy cream 1 teaspoon kosher salt 1/2 teaspoon black pepper 4 ounces white Cheddar cheese, shredded (about 1 cup) 1 cup sour cream 1/2 cup sliced scallions (from 4 scallions)