Cut ACE® White Baguette into 1-inch pieces (about 18 pieces); arrange on a large rimmed baking sheet in a single layer with cut side facing up. Press the centre of each piece down with your thumb to make a small “cup” Set aside.
In a large saucepan, melt butter over medium-high heat, add onions, salt, pepper and mustard powder; stirring often until onions are golden and tender, about 10 minutes. Stir in wine (or whatever liquid you are using) and reduce heat, cover and simmer for about 5 minutes.
Divide warm onion mixture over the baguette pieces and top each with the shredded Gruyere.
Bake in the oven until the cheese is melted and is golden and bubbling, about 10-12 minutes.