Preheat the oven to 400°F and line a large, rimmed baking sheet with parchment paper. Place the halved squash cut side-up on the baking sheet, season with a pinch of salt and pepper, and bake for 20 minutes.
Meanwhile, heat 1 teaspoon of the olive oil in a large skillet over medium heat. Add the onion and garlic, season with a pinch of salt and pepper, and cook, stirring often, until the garlic is fragrant and the onion starts to soften, about a minute. Add the kale and sage leaves and cook, stirring often, until the kale is wilted and dark green, about 2 minutes more. Take the skillet off the heat and let it cool for 5 minutes.
When the mixture has cooled slightly, combine the ground turkey, curry powder, tomato paste, and some salt and pepper in a medium mixing bowl. Use your hands to mix everything together, then add the cooked vegetable mixture and mix everything together again.
Divide the mixture in half, roll each half into a large ball, and stuff inside the pre-cooked acorn squash halves on the baking sheet. Toss the Brussels sprouts in a medium mixing bowl with the remaining 2 teaspoons olive oil and a pinch of salt and pepper, then spread the Brussels sprouts out on the baking sheet with the squash. Bake until the turkey is cooked through, the squash is soft, and the Brussels sprouts are tender and browned, 25 to 30 minutes more.
Serve half the Brussels sprouts with one stuffed squash half.
Leftovers: Let the leftovers (half the Brussels sprouts and 1 stuffed squash half) cool completely before storing them in two separate airtight containers in the fridge.