Remove 1 stalk and leaves from the celery bunch. Dice. Place in ice water and set aside.
Peel outer strings of the remaining celery stalks with a vegetable peeler. Cut ribs into 3-4 inch pieces.
Heat a large skillet on medium-high heat. Add butter and olive oil, allowing butter to melt. Add onion and garlic and stir. Sautee until the onion has softened, about 5 minutes. Add celery, chicken stock, salt, and fresh thyme. Bring to a boil then turn heat down to medium and let simmer uncovered for 15-20 minutes, until celery is just tender.
Transfer to a 9x13-inch baking dish. Pour heavy cream over celery. Sprinkle with Gruyère cheese.
Bake, uncovered at 400 degrees for 30 minutes, until golden and bubbly. Remove from oven and let rest for 10 minutes.
Season with Kosher salt and fresh pepper. Garnish with sprigs of thyme. Sprinkle top with reserved celery & leaves. Serve hot.