Preheat oven to 450°F.* Line a large baking sheet with parchment paper.
Place the Brussels sprouts on the baking sheet.
In a small bowl, whisk together the oil, salt, and pepper. Drizzle over the Brussels sprouts and toss to coat evenly. Spread them back out into a single layer, cut side down.
Roast for 20-25 minutes, or until tender and golden brown with crispy edges. Remove from the oven and set aside.
While the sprouts roast, cook the bacon in a large skillet over medium heat until crispy, about 8-10 minutes. While the bacon is cooking, line a plate with a paper towel.
Place the cooked bacon onto the lined plate. Set aside.
Leave about 4 tablespoons of bacon grease in the pan, carefully removing the rest.
To the pan, add the onion and diced shallots. Reduce the heat to medium-low. Cook until the onions and shallots are caramelized, 15-20 minutes, stirring often.
Mix in the garlic, brown sugar, maple syrup, apple cider vinegar, chili powder (if using), and the cooked bacon.
Simmer over low heat for 5 minutes, stirring occasionally.
Add the roasted Brussels sprouts to the skillet and gently toss until evenly coated in the bacon jam. Cook for 2 more minutes, just to warm through.
Serve warm.