Trim Brussels sprouts by removing outer leaves and cut in half.
Toss the Brussels sprouts, shallots, and garlic in a mixing bowl with 3 tablespoons of olive oil to coat and sprinkle with salt and pepper.
Spread the Brussels sprouts on a sheet pan and place in the oven to roast for 40 minutes. Stir sprouts occasionally to roast evenly.
While the Brussels sprouts are roasting prepare the pancetta (bacon can be substituted if pancetta is not available) by cutting into ¼” diced pieces. Sauté the pancetta with 1 tablespoon of olive oil and cook until crispy and the fat is rendered.
Remove the pancetta from the pan and add the mustard and apple cider vinegar to the fat to make a sauce.
When the Brussels sprouts are finished roasting bring the sauce to a boil while whisking to prevent burning.
Place the Brussels sprouts and cooked pancetta in a stainless mixing bowl and pour the sauce over the Brussels sprouts and toss to coat evenly.
Place in a serving dish and serve immediately or hold in oven under low heat.