Chop the escarole into large pieces and wash well, discarding the hard core and any browned leaves. Drain. Heat a large, high-sided skillet or wok over high heat. Melt the butter and saute the shallot, garlic and red pepper flakes (if using) until aromatic, about 1 minute. Add the bacon and the escarole leaves. Toss and turn quickly as they sizzle and start to wilt, about 2-3 minutes. Add the grapes, salt, pepper and lemon juice and continue to cook until the escarole is just tender and the fruit is warmed through, about 2 minutes more. Turn out on to a platter and serve hot.