Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Toss the cubed butternut squash and the chicken breast with olive oil, sea salt, black pepper, and dried thyme until evenly coated.
Spread the squash and chicken on the baking sheet and roast for 25 minutes, or until the chicken is cooked through and the squash is fork-tender.
Remove the chicken from the pan, slice it into thin strips, and set aside.
While the vegetables finish roasting, heat a medium pot over medium heat and sauté the chopped onion and minced garlic with a splash of water until translucent.
Add the roasted butternut squash and vegetable broth to the pot, bringing the mixture to a gentle simmer for 5 minutes.
Use an immersion blender to process the soup until it reaches a completely smooth and creamy consistency.
Ladle the soup into a bowl, top with the sliced roasted chicken, and finish with a dollop of Greek yogurt.