There's a reason so many restaurants offer portabella "burgers" these days ― the giant mushrooms taste deceptively meaty, with a rich flavor and succulent texture that mimic tender beef steaks.
Truth is, portabellas are nothing more than common brown (crimini) mushrooms that have been allowed to reach adulthood and picked when their caps are 4 to 5 inches wide. Past their prime, they become too woody.
Portabellas are sold whole (with stems), as caps (with stems trimmed), and sliced. Caps are the most practical choice for these recipes. Oven-roasting is a quick way to cook them, plain or filled.