Melt the butter, olive oil on medium heat, add the sliced onions, and cook until translucent and tender. Stir constantly, this should take approx 25 minutes.
Add the wine, thyme, bay leaves, bring to a boil, cook until the wine evaporates, approx 5 minutes.
Roux - either dust the onions with the flour, or make a light roux with the flour and butter (medium heat, melt butter, add flour, stir constantly, cook until caramel coloured brown). If you go with the dusting method, lower the heat to med-low and cook for 10 minutes to clear the raw flour taste.
Add the broth, bring to a simmer, and cook another 10 minutes.
Remove the bay leaves, add salt&pepper to taste.
Ladle into a broiler-safe bowl, top with bread&cheese (see below), and broil until the cheese bubbles and starts to brown.