In a large skillet, melt 3 tablespoons butter over medium-high heat; add 6 chopped shallots and cook, stirring occasionally, until lightly browned, about 10 minutes. Transfer to a large bowl and mix with 2 cups shredded gruyere cheese (about 8 ounces), 1 cup heavy cream, 1/4 cup chopped chives, 1 teaspoon celery salt and 1/4 teaspoon salt. Add 1 thinly sliced 3-pound head cauliflower, then gently toss. Arrange in an even layer in a greased 9-by-13-inch baking dish. Bake in a 375 degrees oven until browned and very little liquid remains, 40 to 45 minutes. Sprinkle with 2 tablespoons chopped chives and let stand for 10 minutes before serving.