In a large pot set over medium heat, heat 1 T oil until shimmering. Add the onions and saute until golden brown.
Whisk in the butter, then the flour. Cook for 1-2 minutes, then deglaze the pan with the wine, using a wooden spoon to scrape up any browned bits from the bottom of the pot.
Slowly stir in the stock; bring to a simmer. Add the whole sprig of thyme. Simmer for 15 minutes, until broth has thickened slightly.
Spoon the soup into bowls and top with a slice of bread (approximately 1/2-3/4" thick and a thick layer of cheese.
Place under a preheated broiler on HIGH for 3 minutes, until cheese is browned. Serve immediately