Preheat oven to 425 degrees F. Lightly oil a baking sheet or coat with nonstick spray. Place cauliflower florets, onions, and garlic in a single layer on the prepared baking sheet. Add 4 tablespoons olive oil; season with salt and pepper, to taste. Gently toss to combine. .Place in oven and roast until cauliflower, onions, and garlic are tender and golden brown, about 30-35 minutes. Cool slightly and blend in Cuisinart. Pour In a large dutch oven, stir in vegetable or chix stock, chopped thyme and bay leaf. Bring to a boil; reduce heat and simmer for about 15 minutes. Remove bay leaf, stir in yogurt; season with salt and pepper, to taste. If the soup is too thick, add more vegetable stock as needed until desired consistency is reached. Serve immediately, or refrigerate and serve the next day.