You can't get much easier than this, unless you want to chuck one of those frozen veggie burgers in the microwave.
Portabella caps (1 for each burger)
ground basil, to taste
1 tbsp olive oil, if pan frying
Whole wheat hamburger buns (I like the jumbo buns for these)
Any condiments you desire. I usually use some combination of the following:
Alfalfa sprouts or broccoli sprouts (I love these, but they do garner strange looks.)
Tabasco sauce (Ok, ok, so I use this on nearly everything.)
Annie's Goddess Dressing (Very tasty and vegan. You can find store locations here.)
Sprinkle soy sauce on portabella caps. Fry 2-3 minute each side in olive oil, or grill 2-3 minutes each side on a grill. dust with ground basil on one side. Assemble bun, portabella, and condiments in standard hambrger fashion and enjoy!