Preheat the oven to 350°F and line a baking sheet with parchment paper.

Melt the butter in a large skillet over medium heat. Add the onion, salt, and pepper, and sauté the onion for about 15 minutes, until very soft and starting to brown.

Trim Brussels sprouts and slice thinly. Add sprouts to the skillet and cook until tender and color brightens, about 10 minutes. Stir in currants.

Arrange baguette slices on baking sheet and top each with gruyere. Add sprouts mixture and bake for 5 minutes, until cheese has melted and bread is crispy. Serve immediately.