Preheat the oven to 375°F and lightly grease a 10.5” by 8” baking dish.
Bring a large pot of generously salted water to a boil. Add the Catelli Gluten Free Penne pasta and cook for 2 minutes shy of the box’s recommended cooking time (6 minutes). Drain and set aside.
Return the pot to the stove and heat the oil over medium low heat. Stir in the onions and caramelize low and slow until sweet and golden brown- about 40 minutes.
Add in the mushrooms and sauté until browned, about 6 to 8 minutes, then add in the garlic and thyme, and sauté until fragrant, another minute. Add a generous pinch each of salt and pepper. Remove the onions, mushrooms and garlic to a small bowl and set aside.
Add the cauliflower and 1 ½ cups of vegetable stock. Cook uncovered for 8 minutes, then add in the dried mushrooms, cover the pan continue to cook until the mushrooms are rehydrated and the cauliflower is very soft. Using a slotted spoon, remove the dried mushrooms and add them to the bowl with the mushrooms.
Transfer the cauliflower plus any residual mushroom poaching stock to a food processor and puree until smooth.
Add a tablespoon of butter and another tablespoon of olive oil to the pan along with the rice flour and stir to make a roux.
Add the milk, the cauliflower puree, nutmeg and a generous pinch each of salt and pepper. Stir in the balsamic and all but ½ cup of the shredded gruyere. Gently stir until fully melted, then season with salt and pepper, to taste.
Add back the onions, mushrooms, pasta, and kale. Transfer half of the mixture the prepared baking dish in a single layer. Top with the reserved 2 ounces (about 1/2 cup) of grated gruyere cheese and then add the remaining pasta and cheese mixture. Set aside.