Trim the root ends from the Brussels sprouts and peel off any yellow or damaged outer leaves. Using a sharp chef’s knife, slice each sprout thinly from top to bottom to create shaved pieces. Alternatively, use the slicing blade of a food processor or a mandoline. Fluff the sliced sprouts with your hands to separate them into shreds.

In a large skillet (a 12-inch cast-iron skillet works well), cook the bacon over medium heat until browned and crisp. Transfer the bacon to a paper towel–lined plate and leave about 1 tablespoon of bacon fat in the pan. If not using bacon, skip this step and proceed with the oil and butter.

To the same skillet, add the olive oil and butter. Warm over medium heat until the butter is melted and foamy. The pan should be hot enough that the sprouts will sizzle when added, but not so hot that the garlic will burn immediately.

Add the minced garlic to the skillet and cook, stirring constantly, for about 30 seconds, just until fragrant. If the garlic starts to brown quickly, reduce the heat to prevent burning.

Add the shaved Brussels sprouts to the skillet. Sprinkle with kosher salt, black pepper, and red pepper flakes (if using). Toss well to coat the sprouts in the garlicky oil-butter mixture. The pan will look full at first, but the sprouts will cook down.

Cook the sprouts for 6–8 minutes over medium heat, stirring every minute or so. Let them sit undisturbed for 1–2 minutes at a time to develop golden, crispy bits on the bottom. They are done when bright green, tender, and lightly browned in spots.

Turn off the heat. Stir in the lemon juice, lemon zest, and Parmesan until the cheese melts slightly and coats the sprouts. Taste and adjust the seasoning with additional salt, pepper, or lemon juice as needed.

Fold in the crumbled bacon and toasted nuts, if using. Transfer the Brussels sprouts to a serving bowl and, if desired, sprinkle with a little extra Parmesan for serving.

Serve warm as a side dish, or enjoy at room temperature as part of a buffet or potluck spread.