Preheat your oven to 400°F and line two baking sheets with parchment paper.
Toss the cubed butternut squash with olive oil, sea salt, and black pepper on one baking sheet.
Place the chicken breast on the second baking sheet and season lightly with a pinch of salt and pepper.
Roast the squash and chicken for 20-25 minutes until the squash is tender and the chicken reaches an internal temperature of 165°F.
While roasting, sauté the chopped yellow onion and minced garlic in a medium pot over medium heat until translucent and fragrant.
Add the roasted squash, chicken broth, and dried sage to the pot, then simmer for 5 minutes to meld the flavors.
Use an immersion blender to puree the soup until completely smooth and creamy.
Shred the roasted chicken breast using two forks and stir the meat directly into the soup.
Ladle the soup into a bowl and finish with a dollop of plain Greek yogurt for extra creaminess.