Heat 3 tablespoons olive oil in a large pot over medium-high heat. Add 2 sliced shallots and cook until lightly browned, about 3 minutes. Add 1 head each torn Tuscan kale and escarole and 1/2 teaspoon kosher salt. Cook, stirring, until wilted, 3 to 5 minutes. Add 1 teaspoon grated lemon zest and a pinch of red pepper flakes; season with salt.