Preheat oven to 425F and line a parchment paper over a baking sheet.
Place kabocha squash on a cutting board. Carefully slice the top and the bottom part of the squash using a sharp knife. Place squash by the flat bottom and slice in half lengthwise. Place squash cut side down and cut again into quarters. Scoop out the seeds with a spoon then carefully remove the skin using a vegetable peeler. Then, cut squash into small, 1/2-inch cubes.
Place cubed squash along with red onion wedges, garlic, and 4-5 sage leaves onto the parchment lined baking sheet. Drizzle avocado oil and season with a pinch of salt and pepper. Toss to coat evenly.
Place baking sheet into the oven to roast for 15-20 minutes or until fork tender and some charred bits start to form.
While waiting for the squash to roast, add butter and the remaining 4-5 sage leaves into a small, light-colored saucepan and simmer over low heat for 5-7 minutes while carefully stirring or swirling the pan, until the specks of milk solids turn dark brown and your house smells like the fall. Remove from the heat to cool slightly.
Onto your serving plate, add the roasted garlic (it should be nice and soft) and mash it with a fork. Add Greek yogurt, a pinch of salt and pepper, and mix well to combine. Top with your roasted squash and onions, a sprinkle of pumpkin seeds, and finally drizzle your sage brown butter over top. Garnish with more sage leaves if desired and enjoy!