Preheat broiler. Make portabella salad: In a medium bowl, combine oil, lemon juice, marjoram, salt, garlic, and pepper. Add portabella; gently toss to coat. Just before serving, mix in zucchini.
Make sandwiches: Split bread and set, cut sides up, in a rimmed baking pan. Broil until golden, 1 1/2 to 2 minutes. Spread cut sides with cheese and broil until melted, about 1 minute.
Arrange mini greens on bottom halves of cheese-covered bread. Add portabella salad, then top with remaining bread halves. Cut sandwiches in half.
*A soft California cheese with a fresh flavor and mild tang; sold at cheese shops and well-stocked grocery stores.
Note: Nutritional analysis is per sandwich.