Heat the olive oil in a large skillet over high heat. Add the chopped pancetta and cook until crispy, 2-3 minutes. Add the onion and garlic and cook until soft, about 3-4 minutes. Add the greens and wilt them down, tossing frequently. Sprinkle in the sugar, toss, then deglaze the pan with vinegar and add in 1/2 cup stock. Reduce heat to low and let the kale mellow a few minutes.