I had some portobello caps leftover from the mushroom stuffing that went with my lamb roast the other day, so I decided to make some portobello burgers. They are so easy and really good! We will definitely be making them again. The marinade is just mustard, balsamic vinegar, and olive oil. You can brush that on and then grill them (or cook them in a pan). I didn't have buns but I had nice bread from yesterday so I used that. With cheese, guacamole, and tomato - voila!