Roast the butternut squash in the oven. Preheat the oven to 425°F (218°C). Slice each butternut squash in half and scoop out the seeds with a spoon. Place each squash half, cut side up, on a large baking sheet (you may need 2 baking sheets) and rub each with a bit of the avocado oil and season with salt and pepper. Roast for 40-50 minutes, or until the flesh is fork tender and lightly browned.
While the squash is roasting, make the filling components.
Make the “sausage.” Place the meatballs in a shallow microwave-safe dish and heat in the microwave for 1 minute to thaw. If not thawed yet, heat for another 30 seconds. Once cool enough to touch, use your hands to break up the meatballs into small chunks. Heat a large frying pan over medium-high heat with a few swirls of the avocado oil. Add the meatball chunks, breaking them up further with a wooden spoon, and cook for 5 to 7 minutes, or until the mixture starts to crisp up and resembles sausage. Transfer the sausage to the shallow dish just used, or a large bowl.1. Note: if you don’t have a very large skillet, I recommend browning the meatball mixture in two batches for the best texture and even browning.
Cook the kale. Heat a teaspoon or two of avocado oil in the same frying pan over medium heat. Add the chopped kale and cook until wilted, 3-5 minutes. Sprinkle with just a tiny bit of salt and pepper, stir to combine, and transfer to the dish with the sausage.
Cook the quinoa. Rinse the quinoa under cold water, if desired (this helps remove any bitter taste some people experience). Add the vegetable broth (or water) to a small saucepan and bring to a boil. Add the quinoa and return to a boil, then reduce the heat to low, cover, and maintain a simmer for 12 minutes, until most of the liquid has been absorbed. Take the pan off the heat and keep the pan covered for 15 minutes. Then fluff with a fork, and transfer to the dish with the sausage and kale.
Make the Creamy Garlic Sauce. Heat the same large frying pan over medium heat with the 3 tablespoons of oil. Once the oil is hot, add the diced onion. Cook for 6 to 7 minutes, until the onion is softened and lightly browned. Then add the garlic and sage (if using), and cook for 2 minutes, stirring frequently, until the garlic is golden.
Reduce the heat to medium-low, then add the flour, stirring constantly for a minute until it’s well incorporated. Gradually pour in the plant milk in stages, whisking after each addition to prevent clumps, then cook for 2 minutes until the sauce has thickened.
Transfer the creamy garlic sauce to a blender and add the 1 teaspoon kosher salt and black pepper to taste. Blend until the sauce is smooth and creamy. Taste the sauce for seasonings, adding more salt or pepper as desired.
Reduce the oven temperature to 375°F (190°C) (or turn off until ready to assemble the stuffed squash).
When the roasted squash is cool enough to handle, use a large spoon to scoop out the flesh from each squash halve, leaving a ½ to ¾-inch -thick border around the sides and bottom.1. Optional: For a nicer presentation, score the flesh in a crosshatch pattern, but be careful to not cut through the skin. Carefully run a spoon from the base of the squash through the neck to pop out the squash cubes, taking care to not break through the skin.
Add the squash flesh from four halves to the sausage-kale-quinoa filling. Sprinkle with a bit of salt and pepper, and mix to incorporate. Reserve the squash from the other two halves for another use (soups, stews, risottos, etc.).
Stuff the filling into the six scooped out squash halves. Pour the creamy garlic sauce on top. Return the squash to the oven. Bake at 375°F (190°C) until hot, about 10 minutes. Serve warm.