Heat butter or olive oil in a large skillet over medium heat.

Add pancetta and cook until crisp, about 5 minutes. Remove with a slotted spoon and set aside, leaving the rendered fat in the pan.

Add the onion and shallot to the pan. Sauté until softened and lightly golden, about 3–5 minutes.

Stir in garlic, Italian herbs, and red pepper flakes. Cook another 30 seconds until fragrant.

Add the shredded Brussels sprouts and season with salt and pepper. Toss well to coat everything in the fat and seasonings.

Cook for 6–8 minutes, stirring occasionally, until the Brussels sprouts are tender and slightly caramelized.

Stir the pancetta back in and cook another minute to combine.

Taste and adjust seasoning if needed.