Line skillet with a thin layer of vegetable broth, and saute garlic and a pinch of red pepper flakes over high heat for a minute or two, until the garlic is golden in color, fragrant, and most of the broth has cooked off. Add enough broth to line the skillet again, then add capers and olives. Saute for a few seconds, then add greens. Stir to combine and continue to cook, using tongs or a spatula to stir the greens around, incorporating them with the other ingredients and to help cook them down. Once greens are dark in color and softer, about 2-3 minutes, turn off heat. Add lemon zest and stir to combine before serving.