These can easily become the star attraction to your next grilled meal! For the grilling itself, build a moderate, medium-heat fire. The veggies come out tender and full of smoky flavor and can be served warm or at room temp. If grill marks do not appear after 5 minutes, adjust either the heat level or the position on the grill. I have made these with chunks of red onion with equally good results. These have great flavor! Interesting factoid: Once the mushroom reaches 4 to 6" in diameter, it is called a Portobello rather than a Portabella. Adapted from Cook's Illustrated magazine, May, 2009. Hope you enjoy.