Roast Sweet Potatoes & Butternut Squash: Line a baking pan with foil or parchment paper (for easy clean up), and spray with cooking spray. Preheat oven to 425 degrees. Spread onions, butternut squash and sweet potatoes onto baking pan, making sure none of them overlap. Sprinkle salt and garlic over vegetables and bake 40-45 minutes, or until vegetables are just roasted.
(Note: You don't have to roast the vegetables first, but this is highly recommended to bring out their best natural sweetness and flavors!)
Slow Cook: Place vegetables (roasted or fresh, whatever you choose), in crockpot along with the celery, chicken stock, (and salt and garlic if you didn't roast it on the vegetables). Cook on high for 4 hours, or on low for 8 hours.
After cooking, pour contents of soup into a blender or food processor, and add 1/2 cup Greek yogurt. Blend/pulse until it reaches desired consistency. Pour into a large serving bowl.
To Prepare the Savory Option: Stir (or blend in) 1/4 cup parmesan cheese, pinch of salt and pepper, or to taste!
To Prepare the Sweet Option: Stir (or blend in) 1 tbs low sugar maple syrup, 1 tsp cinnamon, and a pinch of nutmeg, or to taste!