Use Portobello mushroom caps that measure 3 1/2 to 4 inches across for this recipe. This size is large enough for the filling, given that the caps shrink when grilled. You can use another size mushroom caps; simply adjust the amount of filling used in each.
Garnish these colorful filled mushrooms with green onion fans. To make fans, cut thin lengthwise slits through the onion greens, extending to bulb end. Place the slit onions in ice water until the sliced ends separate and curl.