Slice the onions in half and then in thin strips.
Place butter and olive oil in a stove-top pan or Dutch oven and heat until melted.
Add onions and cook on medium for about 20 minutes, stirring occasionally.
Add beef broth and Dijon mustard.
Add salt and pepper to taste.
Bring it to a boil, cover it, and reduce it to a simmer for 1 hour.
Slice the baguette into long pieces.
Brush the slices of baguette with the olive oil and toast them in a 350° oven until they begin to brown.
Top them each with the gruyere cheese and return them to the oven until the cheese is melted and bubbly.
To serve, scoop soup into large bowls and serve with the cheesy crouton bread.