Fill a Dutch oven or other large ovenproof pot with sliced onions. Dot with 4 tbsp butter and cover. Bake at 350 for 3-3 1/2 hours, checking every hour and stirring well. Near end, check more often.

When onions are golden brown, remove from oven and set on stove top. Cook on low heat, stirring often, until moisture is reduced and onions are very dark brown. Watch that they don't burn.

Add chicken broth slowly, deglazing pan to lift up all the dark bits on the bottom. These are very flavorful (unless they're burned), so don't discard.

Add wine, beef broth (alternatively, add two scoops beef Better Than Bouillon) and 2 cups water). Add bay leaf, sprigs of thyme (tied with twine if you have it--otherwise lay them on top. You'll scoop them out later), Worcestershire sauce, remaining 2 tbsp butter, and salt and pepper. Simmer 20-30 min until thickened. 

While soup is cooking those last 20-30 min, slice baguette and place on baking sheet. Brush with olive oil and bake until crisp, turning once.

Ladle soup in heatproof bowls, top with baguette slices and shredded cheese, and broil on low until cheese is bubbly.