1hr 5min · 10 servings 1 tablespoon olive oil 1 large sweet (yellow) onion, diced 4 cloves garlic, peeled and minced 5 pounds Russet or Yukon Gold potatoes, peeled and cut into 1" cubes 2 teaspoons kosher salt, divided 1/2 cup (1 stick) unsalted butter 1 1/2 cups sour cream, full-fat 2 cups buttermilk or whole milk 2 cups (8 ounces) shredded Gouda cheese 1/4 teaspoon black pepper