Clean portobello mushrooms with a damp cloth. Remove stems using your fingers or a knife. If you'd like, use a spoon to remove the dark brown gills from underneath the mushroom. They don't affect the flavor at all, but they do release dark brown liquid while they cook.
In a mixing bowl, combine olive oil, red wine vinegar, minced garlic, dried parsley, dried basil, dried oregano, a generous pinch of salt, and a dash of pepper. (For the garlic, I like to use a Microplane zester, so that it's almost paste-like & easy to combine with the other ingredients.)
Add the mushroom caps to the mixing bowl. Use your hands to make sure that each cap gets covered front & back with the marinade. If your mushroom caps are especially large, you can add more oil if needed to make sure it's all evenly dispersed.
Put the bowl with mushrooms in the refrigerator, and let them soak in the marinade for 30 minutes to an hour. (If your refrigerator isn't large enough to hold a mixing bowl, the mushrooms can go into a freezer bag instead to marinate.)
Bring a grill, fire pit with grate, or grill pan to a medium high heat. (For a gas grill, I aim for about 500 degrees.)
Put the marinated mushroom caps on the grill, grate, or grill pan. Cook for about 4 minutes on each side & then flip. (Don't flip until the mushrooms have nice grill marks.) Remove from heat & serve.