Melt butter in a heavy soup pot or Dutch oven over medium-low heat with oil. Add onions and cook for 20 minutes.
Stir regularly during the cooking process so onions won’t stick and burn. You want the onions to caramelise and have a nice brown colour.
Continue to stir, remove from heat and pour in wine.
Return to heat, at medium. Add chicken broth and beef broth, allowing it to reduce.
Add garlic, Worcestershire Sauce, salt, pepper, cornflour mixture and reduce heat to low. Simmer for 15-20 minutes.
Butter one side of the bread slices and broil over low heat, allowing bread to brown and become crispy.
When soup is ready, ladle into bowl or ramekin. Place crispy bread on top, and then sprinkle generously with grated cheese and spring onions.