Heat 2 Tbs. of the oil in a wide 8-quart pot with a lid over medium heat until shimmering. Add the pancetta and cook, stirring occasionally, until golden, 4 to 5 minutes. Add the onion and a few generous grinds of pepper and cook, stirring occasionally, until softened and beginning to brown around the edges, 5 to 7 minutes. Add the garlic and cook until the onion is lightly caramelized, 4 to 6 minutes.
Pack the kale into the pot and sprinkle with the marjoram, red pepper fl akes, and 1/2 tsp. salt. Cover the kale and let it wilt for 1 minute. Toss the kale and onions together with tongs, cover, and cook, removing the lid to toss occasionally, until just tender, about 10 minutes (taste a piece to check). Toss with the vinegar, the remaining tablespoon oil and, if necessary, additional salt and pepper to taste.