Regular white mushrooms can be substituted for the baby portabella mushrooms. Any type of lettuce or greens can be used to make this salad.
The beef can be marinated up to 24 hours ahead of time. The other perishable ingredients can be prepared as directed up to 24 hours ahead of time, too; just cover and store each separately.
Baby portabella mushrooms look almost identical to the more common button mushroom in size and shape, but portabellas have a brown exterior. They are very similar in flavor and texture.