Preheat oven to 400°F (200°C). Peel and halve the sweet onions, then slice them into 1/4-inch thick half-moons. Spread the sliced onions evenly on a large baking sheet. Add the unpeeled garlic cloves to the baking sheet.

Drizzle the olive oil over the onions and garlic, then sprinkle with salt and black pepper. Toss gently to coat. Roast for 45-50 minutes, stirring every 15 minutes, until the onions are deeply golden brown and caramelized, and the garlic is soft.

Once roasted, remove the garlic from the baking sheet, peel, and mince. If using butter, melt it in a large Dutch oven or heavy-bottomed pot over medium heat. Add the roasted onions and minced garlic, sautéing for 2-3 minutes to deepen their flavor further. Pour in the white wine (if using) and scrape up any browned bits from the bottom of the pot. Let it simmer for 2-3 minutes until slightly reduced. Add the vegetable broth, fresh thyme sprigs, and bay leaf. Bring to a boil, then reduce heat to low, cover, and simmer for 20-25 minutes, allowing the flavors to meld.

Remove the thyme sprigs and bay leaf. For a smoother soup, use an immersion blender to blend a portion of the soup (about half) directly in the pot, or transfer desired amount to a regular blender (carefully, in batches). Blend until desired consistency, then return to the pot. Stir in the heavy cream and butter (if using) until melted and incorporated. Taste and adjust seasonings (salt and pepper) as needed. Ladle the hot soup into bowls. Garnish with fresh chopped chives or parsley and serve immediately with crusty bread or croutons.