Preheat your oven to 350 degrees F. Line a baking sheet with parchment. Add the yellow onion and garlic, cut side up, along with the carrots and thyme. Drizzle everything with about a tablespoon of olive oil and add a few pinches of salt and rounds of pepper. Flip the onion and garlic over so the cut side down hits the baking sheet. This will make it so they steam and roast slowly versus burn.
Add the acorn squash and kabocha squash, cut side down, to the baking sheet. Transfer to the oven to bake until tender, about 45 minutes. Remove the baking sheet from the oven and place on the counter to chill for about 10 minutes. You really just want it to cool until you can handle it. Peel the onion and discard the skins.
To a blender, add half of the vegetable stock or chicken stock. Scoop out the flesh of the acorn and kabocha squash and add it to the blender. And then add the removed thyme leaves, carrots, peeled onion and squeeze in the roasted garlic cloves. Lastly add the sour cream or créme fraîche and a few pinches of salt and a few rounds of black pepper.
Blend until very smooth, being sure to allow some of the steam out, along the way. Continue until very smooth. Add the remaining stock and blend it up once again. Give it a taste and adjust the salt and pepper to taste. If you want it creamier, feel free to add a tablespoon or two more of sour cream or créme fraîche.
If the soup is cold, feel free to add it to a pot or saucepan and warm it up until hot. Divide it amongst bowls and drizzle with créme fraîche, crispy sage and a drizzle of good olive oil.