Add butter, oil, onions, thyme, garlic, and salt into a large pot. Saute onions in butter and oil with lid on for 10 minutes. High heat. Stir occasionally. Cook until softened.
Then take off lid, add sugar, and continue to cook on high for another 15-20 minutes. Cook onions until they become a medium brown color.
When onions are brown, add in sherry. Then add in stock, beef base, and stir gently, rubbing off the flavorful bits from the surfaces of the pan. Simmer another 20 minutes.
Preheat oven to 350 degrees. Place sliced baguettes onto a sheet tray and place inside oven. After toasting 5 minutes, flip and top with cheese. Set oven to low broil and place toast into oven for another 3 minutes until cheese is melted and slightly brown.
Serve soup with cheese toast.