Cut the onions in half and peel. Then cut them into ¼ inch slices. Do not slice the onions too thin or they will disintegrate during the caramelizing process.
Heat the olive oil and butter in a large pot over medium heat. When the butter is melted and foaming, add the sliced onions.
Cook the onions for approximately 45-50 minutes. Stir the onions only occasionally at first. As they begin to turn a soft caramel color (after 15 minutes or so) start stirring them more often to keep them from burning.
After approximately 25 minutes stir the onions almost constantly and gently scrape any bits off the bottom of the pot.
Add the white wine to the pot, stir while gently scraping any remaining bits off the bottom.
Cook the wine with the onions for approximately 2 minutes or until most of the liquid has cooked out.
Add the chicken broth, bay leaf, thyme, salt and pepper to the pot and stir well.
Raise the heat to medium-high and bring the onion soup to a simmer. Lower the heat to medium-low, cover and cook for 20 minutes, stirring occasionally.
Keep the onion soup at a simmer, if it’s boiling too vigorously, lower the heat.