Find a baking dish that will fit both mushroom caps (tightly line dish in foil to make clean up easier if desired). Add all ingredients except mushroom caps and stir to create marinade.
Add mushrooms and rub sauce onto them. Let sit for 15 minutes, stem side down.
Once 15 minutes is up, rub mushrooms again with sauce before placing dish in the oven.
Bake for 15 minutes. Remove and turn mushrooms over. If there is any sauce left in the pan, spoon it over the mushrooms. Continue baking another 5-15 minutes (exact cooking time will vary based on mushroom size).
Remove mushrooms and let sit for 5 minutes before slicing and serving.
Choose big, fresh portobellos and if you have smaller ones, you can always double the sauce and cook 3 or 4 portobello caps to make sure you get the same number of servings. If yours are bigger though or if you're using three smaller portobellos, you might want to scale the sauce up a bit. The risk of under-saucing is more dire than the risk of over-saucing with this recipe.
To save your pan, you can line it with foil or parchment paper. Or if you don't line the pan, be sure to soak it immediately.
Letting the mushrooms rest for a few minutes before slicing retains juices and flavor.