We used to have a marinade we would use for London broil overnight the. Sear and grill to med rare/med. Slice across at 1/4". Awesome hot, great for sandwiches the next day. Enjoy with the mushrooms. Looks great.
Discussion
I used to marinate it too but then tried koshering salt and fresh course ground tellicherry black pepper. Coated and put in fridge in the morning, pull out and hour or so before you want to grill.
Much easier and the flavor is simple but amazing. I use a steak seasoning from the Asian market on the portobellos. It has fennel and some other interesting stuff, I don't know but it's good, along with a good drenching of olive oil. Grill till tender.