Heat the butter and oil in a skillet over medium-low heat. Add in the sliced onions. Cook, stirring frequently, for around 35–45 minutes until golden and soft.
Add the Worcestershire sauce and stir to combine. Season with salt and pepper.
Lay the toasted bread onto a baking sheet and evenly divide the onions between the slices, spreading into an even layer.
Top with the grated Gruyere cheese.
Broil for a few minutes until the cheese is melted. Serve with chopped chives and a crisp green salad.