Bitter is Better: People are divided into two camps when it comes to bitter leafy greens. Some like it when a vegetable like broccoli rabe and mustard greens bites back, and the rest are delicate snowflakes who just can’t handle it. If you’re a newbie or in the latter group, give this dish a try over the winter. Frost makes greens and cruciferous vegetables sweeter, and the garlicky oyster sauce is good on practically anything.
Trim the bottom of the stems from the mustard greens (just trim the tough, dried out bottom parts; keep most of the stems). Separate the stems from the leaves of the greens. Cut the leaves and stems into 2-inch pieces.
Stir together the cornstarch, oyster sauce, and 6 tablespoons of water in a small bowl and set aside.
Cook the pancetta in a large nonstick skillet over medium heat, stirring occasionally, until golden, about 8 minutes. Transfer pancetta with a slotted spoon to a paper-towel-lined plate. Increase heat to medium-high, add the garlic to the rendered fat in the pan and cook, stirring occasionally until barely golden, just a few seconds. Add the stems and leaves to the skillet and cook, tossing to coat in the oil, until the greens wilt, about another 2 minutes. Give the cornstarch mixture a quick stir and add it, the salt and the pancetta to the greens and mix well. The sauce should thicken up slightly and coat the greens. Adjust seasonings. Drizzle with sesame oil and serve with rice if you like.