Washington’s Rich Brown seasoning and broth does a pretty good approximation of beef flavor, and it’s vegetarian. They have ‘Golden’, too, that is a yummy chickenish flavor.
I’m not a vegetarian, but I love how they taste and use them all the time.
I bet it would make a pretty good marinade.
Octavia: perhaps it would save a lot of people’s time if you could just clear up whether it was a strictly vegetarian recipe you were after, or whether we can legitimately keep on suggesting that you add cow parts to achieve the desired effect. 
To do it vegetarian, just throw some steak sauce on them.
Jamie Oliver, on his show, Oliver’s Twist, recently did a “Mushroom Sarnie”, which he claimed tasted just like a steak sandwich. They certainly looked like steak sandwiches when he served them up, with the brown juice drizzling down and soaking into the bread.
BTW, on reviewing this recipe, I see that it lists “white mushrooms” in the ingredients, but Jamie most definitely used some big honkin’ portobellos in the TV show.
I am not a vegetarian - so feel free to recommend meat products. I will say that I am hoping to avoid purchasing beef as it can be $pendy.
I must say, many of the suggested procedures above sound really good!
So thanks all (except for you snide remarkers, who I’ll be chucking through plate glass windows as soon as time permits 
)
Grilled mushrooms with caramelized onions and roasted peppers on really good bread. I wish I liked mushrooms- that sounds good…