Preheat oven to 450°F. Line a baking sheet with parchment paper, and set aside.
Place squash in a medium-sized bowl and toss with 1 tablespoon of olive oil. Arrange on a baking sheet and roast for 15-20 minutes, until tender.
While the squash is cooking, Heat 1 tablespoon of olive oil in a large stockpot. Sauté onion, celery, garlic, and apple over medium heat for about 5-7 minutes until onion is softened.
Add the squash, vinegar, stock, and salt. Bring to a boil, reduce heat, and simmer uncovered for 15-20 minutes.
Puree with a hand-held emulsifier or in the blender in batches until desired consistency.
Serve and top with a dollop of Greek yogurt, chopped pecans, and fresh parsley.