Sometimes you're throwing a taco party and you need a vegetarian option. Sometimes you just don't feel like eating meat.
Since a proper taco is never overstuffed, one decent-sized portobello should be enough for three servings. Multiply accordingly, and drop your mushrooms into a big Ziploc bag with enough salsa verde to cover. Let everything sit in your fridge for a couple of hours.
Remove, season with just a touch of salt and cracked black pepper, then lay onto a grill (or a grill pan on your stovetop) over medium to medium-high heat for a few minutes on each side. Cut into thick slices, and you'll have a nice, earthy taco base.