Set the oven temperature to 350°F. Cut the cabbage into 8 wedges. In a large, heavy skillet, heat the olive oil over medium-high heat, and add the cabbage. Sear it on both sides until the cabbage gets as much color as possible, about 1 minute per side. Use a spatula to transfer the cabbage to a rack placed over a rimmed baking sheet. Sprinkle the cabbage pieces with salt and pepper. Pour the Guinness into the bottom of the baking sheet, then carefully transfer the pan to the oven. (The beer will gently steam the cabbage and infuse it with stouty essence). Bake the cabbage for 20 minutes, or until tender and deeper brown.