Melt butter in a Dutch oven over medium-high heat. Add onions and baking soda; cook 15 minutes, stirring with a wooden spoon every few minutes. Continue cooking about 10 minutes, scraping the bottom of the pan often and lowering the heat, as needed, until the onions are very soft and a deep golden brown.

Stir in flour and cook 1 minute. Add wine and sherry, if desired, and cook until evaporated. Stir in broth, thyme and bay leaves; bring to a simmer. Reduce heat to medium and simmer 30 minutes.

Meanwhile, preheat broiler with rack in the upper third of the oven. Line a baking sheet with aluminum foil and coat with nonstick cooking spray. Place baguette slices on the baking sheet, arranging slices touching each other in groups of 2 or 3 to fit the bowls. Toss cheese with parsley; sprinkle evenly over bread slices.

When the soup is ready, remove thyme stems and bay leaves. Broil toasts 2 to 3 minutes until cheese is melted and browned in spots. Ladle soup into 6 bowls and transfer a grouping of cheese toast to top each bowl.