Heat oven to 425 degrees F. Line a rimmed baking sheet with parchment paper.

Slice Brussels sprouts in half lengthwise. In a medium bowl, toss Brussels sprouts in oil and season with salt and pepper. Spread on prepared baking sheet in a single layer and roast until slightly golden and tender, 20 to 25 minutes.

While Brussels sprouts are roasting, heat a medium sauté pan on medium-high heat. Add bacon and cook until well browned and crispy, draining excess fat as it cooks if needed, about 8 minutes. Remove to a paper towel-lined plate to cool.

In a small saucepan over low heat, melt 1 tablespoon of butter. Add chopped hazelnuts and 1 teaspoon of salt and cook, stirring often, until golden and fragrant, about 3 minutes. Add honey (it will bubble up) and continue stirring until slightly thickened, about 2 minutes more. Turn off the heat, add apple cider vinegar, orange zest, red pepper flakes if using, 1/2 teaspoon of salt and remaining tablespoon of butter. Stir again until butter is melted and sauce is well combined.

Transfer Brussel Sprouts to a large serving bowl, toss with honey-hazelnut glaze and sprinkle with bacon. Serve warm.