You’ll love the flavors in this satisfying side dish. The onion adds to the slight sweetness of the celery root, while the Gruyere adds to its nuttiness. This is a decadent dish that smells delicious and comforting as it bakes. Just be sure to let it rest after it comes out of the oven. Cooling slightly will allow the cream to finish thickening. Celery Root and Onion Gratin Serves 4 to 6 1 tablespoon butter 2 medium celery roots, peeled, halved, and thinly sliced (2 pounds) 2 cups thinly sliced onion (about 2 medium onions) 1 tablespoon finely chopped garlic (about 2 cloves) 1 teaspoon salt 1/4 teaspoon pepper 1 cup whipping cream (35 percent fat) 1 cup grated Gruyere cheese Preheat oven to 400 degrees F. Butter an 11-by-7-inch glass baking dish. Mix together celery root, onion, garlic, salt, pepper, and whipping cream in a large bowl. Pour celery …