Add 1/2 cup soy sauce to 5-6 quart of boiling water. Quickly blanch the cauliflower, drain well and transfer to a buttered baking dish. Meanwhile melt 2 tablespoons butter in a saucepan over medium heat, stir in flour. Cook roux for 3 minutes, add soymilk, whisking to blend. Bring to a boil, reduce heat and simmer until it thickens approx. 5-7 minutes. Remove sauce from heat, add shredded cheese, celery, onion, salt and pepper, stirring to combine. Pour cheese sauce over cauliflower. Melt remaining butter in bowl in the microwave, add Panko bread crumbs. Sprinkle crumb topping over cauliflower. Bake in a 450° for about 10 minutes until topping browns slightly.