Preheat your oven to 400°F and line a baking sheet with parchment paper.
Toss the cubed butternut squash with olive oil, sea salt, and black pepper, then spread in a single layer on the baking sheet.
Roast the squash for 25 to 30 minutes until the edges are golden brown and the flesh is tender.
While the squash roasts, poach or pan-sear the chicken breast until it reaches an internal temperature of 165°F, then shred it using two forks.
In a large pot over medium heat, sauté the diced yellow onion and minced garlic until translucent and fragrant.
Stir in the chopped fresh sage and cook for one minute to release the oils.
Add the roasted butternut squash and vegetable broth to the pot, bringing the mixture to a gentle simmer.
Use an immersion blender to puree the soup until it is completely smooth and silky.
Fold the shredded chicken into the soup and heat through for 2 minutes.
Ladle the soup into a bowl and finish with a dollop of plain Greek yogurt.