Slice each Brussels sprout in half and remove the core. Gently pull the layers of leaves apart.
Heat a large sauté pan and add the olive oil and pancetta. Render the fat from the pancetta, until it is lightly crispy. Remove the pancetta and reserve.
Add the onions and cook until translucent, about 5 minutes.
Add the butter and swirl to melt. Add the sprouts and water.
Cook over medium heat, tossing to coat, until leaves are tender and bright green, about 6 to 8 minutes. Adjust seasoning, fold in remaining pancetta, and serve immediately.
Sautéed Brussels Sprouts with Beurre Noisette: Omit the pancetta, decrease the amount of olive oil to 2 Tbsp/30 mL, and increase the amount of butter to 2 oz/57 g. Cook the entire amount of butter separately. As the sprouts are finishing cooking, heat a small sauté pan over medium heat. Just before the sprouts are done, add the whole butter to the pan; if it sizzles, remove the pan from the heat and swirl until the sizzling stops. Place the pan back on the burner and gently cook the butter until it has a nutty smell and is a medium dark brown color. Toss the cooked Brussels sprout with the brown butter and serve.