Heat oil in a large skillet or dutch oven over medium-high heat. Cook onion until soft, about 6 minutes, then add ginger and garlic. Stir in ground pork, tofu (if using), salt, brown sugar, and lime leaves. Cook until fat renders and pork and tofu are nicely browned (it'll take a while - about 10 minutes past the point that the pork is actually cooked through). Stir in soy sauce, juice of 1 1/2 limes (reserve the last half-lime for serving), and cilantro.