Heat oil in a large Dutch oven on medium. Add garlic, thyme, thinly sliced zest, and red pepper flakes and cook, stirring, until garlic is golden brown, about 2 minutes. Add escarole, in 2 batches if necessary, and 1/2 teaspoon salt and cook, stirring occasionally, until escarole is beginning to wilt, about 3 minutes. Reduce heat to medium-low, fold in beans, and cook until escarole is tender and beans are heated through, 2 to 3 minutes more.