A portobello mushroom is a mature brown or button mushroom. The cap has opened and flattened resulting in a mushroom with a large flat cap resembling an umbrella.

Picture of a really large portobello mushroom. Photo by Chameleon found at Wikimedia Commons.
The cap is thick and fleshy. When grilled, it exudes a wonderful smoky aroma and a tender chewy texture which resembles meat.
It is frequently stuffed because of the large cap but it works equally well in-between bread. Here's a nice burger recipe:
Ingredients4 portobello mushrooms
Olive oil
Salt and pepper
Roasted red peppers
4 cups of salad greens
4 generous slices of cheese
1/2 cup basil pesto
1/4 cup mayonnaise
4 round bread rolls
- Remove the stems from the portobello mushrooms and clean them by scrapping off the gills and gently wiping the caps with a damp cloth or kitchen towel
- Mix the basil pesto and the mayonnaise together
- Heat up the grill or skillet and oil it lightly with cooking oil or butter
- Cut the bread rolls halves and place each half cut side down onto the grill and toast them lightly.
- Spread the basil and mayonnaise mix on the sides of rolls that were grilled.
- Brush the mushrooms on both sides with the olive oil and sprinkle with some salt and pepper. Rub a little if you want.
- Place the mushrooms onto the grill and cook till tender. Turn them over and grill the other side.
- When the mushrooms are cooked through and tender, place a slice of cheese over the mushroom.
- When the cheese has melted slightly, remove the mushrooms and place one on each grilled roll
- Layer with the roasted red peppers and salad greens.
- Cover with the other half of the roll and serve hot.
Recipe link: Portabella mushroom burger