Heat a large saute pan or cast-iron skillet over medium-high heat. Add the bacon and cook until browned and crisp. Add the onions and cook 2-3 minutes more or until softened. Transfer the bacon and onions to a paper-towel lined plate to drain, reserving the bacon fat in the pan.
Take the slices of Brussels sprouts and, using your fingers, separate the layers into shreds.
Place the Brussels sprouts shreds into the hot pan with the bacon fat. Season with salt and pepper. Stirring frequently, cook the Brussels until they are crisp-tender and bright green, 5-10 minutes. In the last 3 minutes of cooking, add the bacon, onions, and hazelnuts to the pan, tossing to combine. Taste to adjust seasoning. Serve promptly.