Preheat oven to 300 degrees.
Heat a thick-bottomed skillet over medium heat. Add the sliced butter (so it melts more evenly) whisking frequently. Continue to cook the butter. Once melted the butter will foam up a bit, then subside. Watch the butter carefully as lightly browned specks begin to form at the bottom of the pan. Smell the butter; it should have a nutty aroma. Remove from heat and place on a cool surface to help stop the butter from cooking further.
Using a cast iron skillet or non-stick skillet, pour in your browned butter and add the Brussels sprouts, cut side down. Cook until they are dark brown and well caramelized on the cut surface. Season them with salt and pepper and then transfer to a baking dish or prepared cookie sheet and place in the oven to roast for about 7-8 minutes, depending on the size of your brussels sprouts. You just want to ensure they are cooked all the way through but not overcooked.
While the Brussels sprouts are roasting in the oven, pan fry your pancetta over medium-high heat until the desired crispiness has been reached. Remove from pan and set aside.
In a small skillet, heat about 1/4 inch of the oil until shimmering. Add the sliced shallots and fry over moderately high heat, stirring occasionally, until golden brown and crisp, or for about 3 minutes. Using a slotted spoon, transfer the fried shallots to paper towels to drain.
Once the Brussels sprouts are ready, remove from the oven. To plate: plate your Brussels sprouts, next, add the pancetta if using, then sprinkle the hazelnuts, grated cheese and a few fresh leaves of thyme over the Brussels sprouts. Lastly, top with a mound of the crispy shallots and very lightly drizzle (you don't need much) the aged balsamic vinegar on just before serving.