You don't have to grill the mushrooms. You can saute them in butter or olive oil or a combination of both or broil them instead.

Or roast in the oven.
Remove the gills and cut the cap into 1/2 inch thick slices, wash and remove the seeds from a red pepper.....and while you are at it cut a peeled sweet onion into 1/4 inch slices.
Place the veggies on a cookie sheet, drizzle with olive oil, sprinkle with salt, pepper and a few dashes of balsamic vinegar.
Place into a 425 oven for about 20 minutes and put into a hard roll or on a slice of good Italian bread.
You can also sprinkle the veggies with grated garlic and oregano....and black pepper!
Linda C

My recipe is similar to Linda's. It's adapted from a recipe I saw in the Washington Post years ago. You can slice them (after they marinate) & saute them in a skillet instead of grilling. I use a light olive oil. Ann

Grilled Portobello Mushroom Sandwiches

4 large portobello mushrooms (6-10" across)
½ cup olive oil
2 cloves garlic, minced
1 tsp. salt
½ tsp. fresh ground black pepper
2 tsps. Worcestershire sauce

Cut the stem off each mushroom near the base. Clean out the gills. Whisk the olive oil, garlic, salt, pepper and Worcestershire sauce in a small bowl and spoon it evenly into the cleaned out gill area of each mushroom. Refrigerate for 1 hour. Brush the grill rack with oil. Grill the mushrooms top-sides down over low heat. Close the grill lid and cook for 10 minutes. Pierce the thickest part of the largest mushroom with them tip of a small sharp knife. If it feels tender, remove all the mushrooms.

Slice the mushrooms and serve on Italian peasant (ciabatta) bread with aioli, slices of red onion and slices of roasted red pepper. If I have fresh basil, I like to slice it and add it to the sandwich. I also like to grill the ciabatta slices.
Aioli
1 garlic clove
2/3 cup mayonnaise
2 tsps fresh lemon juice
Some zest from the lemon
½ tsp dry mustard
pinch or more of cayenne pepper
Blend in food processor and then taste for seasonings.