Toss the sweet potatoes and onions with the vegetable oil, salt, and pepper.
Transfer vegetables to a baking sheet (or two) depending on the size of your baking sheets and/or oven. Keep the vegetables in a single layer. Place two sprigs of rosemary on each baking sheet, or all 4 sprigs on the same baking sheet if you're only using one.
Bake for 45 minutes, turning once at the 25 minute mark.
Transfer the roasted sweet potatoes and onions to a heavy-bottomed soup pot. (Discard the rosemary sprigs.)
Add the garlic, fresh thyme leaves, and vegetable stock. Stir well to combine all ingredients.
Place a lid on the pot and bring to a low boil over medium-low heat for 30 minutes, stirring occasionally.
Use a hand-held immersion blender, or a counter-top blender, to puree the soup. Taste for seasoning and add more salt and pepper if needed.
Serve with a tablespoon of light sour cream, toasted croutons, and a sprig of fresh thyme for garnish.