In a large skillet, heat the kale and water over medium-high heat, covered, for about 10 minutes, or until all the water is absorbed. Remove the kale from the skillet and set aside in a large mixing bowl.
In the same large skillet, heat 2 tablespoons of the olive oil over medium-high heat. Add the leeks and onions and cook, stirring occasionally, for 15 minutes, until the onions are beginning to turn translucent and the leeks are starting to soften. Add the mushrooms and cook for 5 minutes. Season to taste with salt and pepper. Push some of the vegetables out from the middle of the skillet to expose the bottom of the pan. Add the remaining 1 tablespoon of olive oil, garlic, thyme, and rosemary, and cook for 30 seconds. Give it all a good stir and remove from the heat, adding the vegetable mixture to the kale in the large mixing bowl. Add the cooked rice and quinoa and mix together.
In a medium-sized saucepan, heat the butter over medium heat. Once the butter is melted, whisk in the flour to form a paste. While whisking constantly, add the vegetable stock, milk, and heavy cream. Continue to whisk constantly, until the sauce starts to boil, about 7 minutes.
Reduce the heat to low and simmer, stirring occasionally, until sauce begins to thicken, about 10 minutes.
Remove the sauce from the heat and stir in the nutmeg and the Gruyère cheese until blended. Salt and pepper to taste. Pour the sauce in the large mixing bowl with the wild rice, quinoa, and vegetables, and mix together until blended. Transfer the mixture into a greased 13-by-9 inch baking dish.
Preheat the oven to 400 degrees Fahrenheit.
In a large bowl, whisk together the flour, baking powder, baking soda, sugar, and salt. Work the butter into the mixture with either a pastry cutter or your fingers until the butter is incorporated and the dough has pea-sized lumps.
Stir in the Gruyère and the buttermilk just until the dough comes together. Drop the biscuit dough onto the casserole filling in 6-8 large mounds, leaving a bit of space between each biscuit.
Bake the casserole for 35-40 minutes, until the biscuits are golden brown on top and the filling is bubbling. If your biscuits start to brown too quickly, cover the dish with foil and continue to cook. Let the casserole stand 10 minutes before serving.