Preheat your oven to 400°F 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 in a single layer on the baking sheet and roast for 25-30 minutes, or until the squash is tender and the chicken is fully cooked.
While roasting, sauté the chopped yellow onion and minced garlic in a large pot over medium heat with a splash of water until translucent and fragrant.
Remove the chicken from the baking sheet and set aside to rest; transfer the roasted squash into the pot with the onions.
Pour in the vegetable broth and bring to a gentle simmer for 5 minutes to meld the flavors.
Use an immersion blender to purée the soup until it reaches a smooth and velvety consistency.
Dice the roasted chicken into bite-sized pieces and stir them back into the soup.
Remove from heat and swirl in the Greek yogurt for a rich, creamy finish before serving.