Peel, trim the ends and halve the squash lengthwise.Scoop out the seeds and cut the squash into cubes; you'll need 2 1/2 to 3 cups of squash.
Toss the squash with 1 tablespoon of the coconut oil in a 9-by-13-inch metal baking pan. Roast for 30 to 35 minutes.
Cook the pasta according to package instructions in a large pot of boiling salted water.
While the squash is roasting, heat another tablespoon of the coconut oil in a medium saucepan over medium heat. Add the garlic and oregano, and saute until the garlic begin to brown, 2 minutes. Add 1 1/2 cups of the coconut milk and the arrowroot powder, and whisk well, making sure there are no lumps. Add the nutritional yeast, lemon juice, mustard, pepper and truffle salt, and cook, stirring vigorously, for 3 to 5 minutes, until the sauce has thickened nicely. Remove the saucepan from the heat. Set aside.
When the squash is done, remove it from the oven and let it cool for a few minutes. Lower the oven temperature to 350 degrees F. In a high-speed blender or food processor, combine the roasted squash and the remaining 1 cup coconut milk, and puree until very smooth, 30 to 45 seconds. Add the squash mixture to the saucepan and whisk it together with the sauce until well combined. Mix together the cooked pasta and the squash sauce in a large baking dish. Scatter the collard greens over the top, sprinkle with some smoked paprika, and top with the pecans. Bake for 20 minutes.
Heat the remaining 1 tablespoon coconut oil in a medium skillet over medium-low heat. Working in batches, fry the sage leaves until crispy, flipping once, about 1 minute total per batch. (Watch carefully to ensure the sage doesn't burn; you want it just slightly browned.) Drain the sage on paper towels and season lightly with truffle salt. When the mac 'n' cheese is done baking, garnish it with the sage leaves and serve.