Preheat oven to 375°. In a large skillet over medium heat, melt butter. Add onion and cook until very soft, about 8 minutes. Stir in garlic and cook until fragrant, about 1 minute. Add brown sugar, maple syrup, apple cider vinegar, and chili powder, then season with salt and pepper. Cook until mixture has reduced and onions are jammy, about 10 minutes more. Remove from heat.
On a large rimmed baking sheet, toss brussels sprouts with olive oil and season with salt and pepper. Cook until just tender, about 20 minutes. Let cool slightly.
In a medium skillet over medium heat, cook bacon until crispy, about 8 minutes. Drain on paper towels, then chop into squares.
To assemble slider, top the cut side of one brussels sprout half with a piece of bacon. Place a piece of brie on top then dollop a small amount of onion jam on top of brie. Place a second brussels sprout half on top, cut side-down, then secure the sandwich with a toothpick. Repeat with remaining ingredients.
