Preheat the pellet grill to 185°F (85°C) and activate the Super Smoke setting if available.

Place the 3 lb Spanish onions directly on the grill and smoke for 60 minutes.

Remove the smoked onions and let them cool briefly until safe to handle.

Peel and thinly slice all smoked Spanish onions.

Heat the large pot over medium heat and melt the 6 tbsp butter.

Add the sliced onions and 2 tbsp of the fresh thyme leaves to the melted butter.

Season the onions with about 1 tsp sea salt and cook for 40 to 45 minutes.

Stir the onions occasionally until they are deeply golden, soft, and caramelized.

Add the minced garlic and cook for 1 minute while stirring.

Pour in the 1 cup sparkling wine and scrape the browned bits from the bottom of the pot.

Simmer until the sparkling wine is almost entirely evaporated, about 3 to 5 minutes.

Add the 2 qt beef stock and 2 bay leaves to the pot.

Season the soup with the remaining sea salt and cracked black pepper.

Bring the soup to a gentle simmer and cook for 30 minutes.

Preheat the oven broiler and arrange the 16 baguette slices on a baking sheet if they are not already toasted.

Toast the baguette slices under the broiler until crisp, watching closely to prevent burning.

Ladle the hot onion soup evenly into 8 oven-safe soup bowls.

Top each bowl with 2 toasted baguette slices and a generous portion of grated Gruyere cheese.

Sprinkle the remaining fresh thyme leaves and cracked black pepper over the cheese.

Broil the bowls until the Gruyere melts, bubbles, and turns golden brown, about 2 to 4 minutes.

Garnish the soup with the minced fresh chives and serve immediately while hot.