In a Dutch Oven or large soup pot, melt the butter in the olive oil over medium-high heat.
Add yellow onions, red onions, and salt to the pot, and cook, stirring constantly, until the onions begin to soften, approximately 5 minutes.
Stir in the sugar and continue to cook until the onions are richly coloured and carmelized, about 20 minutes.
Add the wine, stock, and thyme to the pot and simmer for an additional 20 to 60 minutes, stirring occasionally.
Cover and keep warm while you prepare the bread.
Just before serving, preheat oven broiler.
Arrange bread slices on a baking sheet and broil for approximately 2 minutes, turning once until toasted on both sides.
Remove bread from the oven and put aside.