To make the wild rice:
-Bring 1 quart of water to a boil in a saucepan.
-Add 1 cup wild rice and ½ tsp sea salt.
-Lower the heat and cover, cooking for 45-50 minutes until the rice is tender but still has a little bite.
-Drain off any excess water and let stand for 5-10 minutes and then fluff with a fork.
For the sauce:
Cook ¼ cup chopped onions in 3 TBS butter over low heat for 3 min.
-Stir in 2 TBS flour and cook for 2 min.
-Heat 1½ cups whole milk until just boiling and then add to the butter all at once.
-Cook until thickened, about 10 min.
To assemble: Preheat oven to 400 degrees.
-Lightly butter a baking dish.
-Melt 1 TBS butter in a medium skillet over medium heat.
-Add 1 celery root peeled and grated, the juice of 1 lemon and some salt and pepper.
-Cook until tender, about 5-7 min.
-Combine the rice, celery root, sauce, ½ cup grated Gruyere cheese and ¼ cup grated Parmesan.
-Turn into the buttered dish and bake for 25 minutes until firm and lightly browned.