Cut the cabbage into quarters. Cut off and discard the hard core from each wedge. Slice the cabbage into ½-inch-wide strips. Set aside.
In a deep 12-inch skillet, melt the butter over medium heat. Add the onion, carrots and bell pepper. Cook, stirring often, until softened, about 5 minutes. Stir in the cabbage and potatoes.
Add water and season with salt and red pepper flakes to taste. Bring to a boil. Reduce the heat to medium-low and cover.
Cook until the potatoes are tender and most of the liquid has evaporated, about 20 minutes.
Transfer to a serving dish and sprinkle with the cilantro, if desired.