Here's what I do at work:
First, make the beer batter, preferably with a medium-dark brew. For one bottle, add enough flour to make a batter as thick as, say, pancake batter. Add some salt and pepper, and a teaspoon or two of malt powder (YES!) to the batter, and stir.
Brush the mushrooms clean. Dredge in flour, shaking off the excess. Dip into the batter, and drop, one-by-one, into the fryer. I usually take the second fryer basket (I'm using a commercial deep-fryer), and push the mushrooms down into the oil. Cook for a couple minutes. Remove the second basket, and make sure the mushrooms haven't stuck to each other. Cook for one more minute (I like my mushrooms rather well-cooked), remove from the oil, and drain for a minute. Plate, and season to taste with a little salt and pepper. We serve ours with Ranch.
Peace, Love, and Vegetable Rights!
Eat Meat and Save the Plants!