Those big, juicy-looking portobello mushrooms at the market are fantastic when they're simply broiled or grilled. They can be eaten on their own or act as a patty for a vegetarian alternative to the hamburger. Either way, this recipe is quick and easy. Large portobellos are meaty and filling. For the broiler, all they need is a marinade of garlic, olive oil, vegetable broth, and balsamic vinegar. Most of your time will be spent waiting for them to marinate. After that, it takes just a few minutes on each side to grill them to perfection. This same recipe can be used on your outdoor grill if you feel like cooking out.