Preheat broiler. In a 10-inch nonstick oven-going skillet heat olive oil and butter over medium heat. Add onion; cook and stir 3 minutes or until softened. Add zucchini and 1 tsp. of the marjoram; toss gently to coat with butter mixture. Spread zucchini in a single layer in skillet; cook 10 to 15 minutes or until tender and browned in spots, turning gently once or twice.
Meanwhile, in a medium bowl whisk together eggs, salt, black pepper, and the remaining marjoram. Pour egg mixture into skillet; top with feta. Reduce heat to low; cook, without stirring, 12 minutes or until top is just set.
Broil frittata 4 to 5 inches from heat 3 to 4 minutes or until top is set and browned. Remove from oven; let stand 1 minute. Transfer frittata to a serving platter; let stand 10 minutes. Top with additional feta cheese and marjoram. Serve with ciabatta slices spread with mayonnaise.
TIP: In Italian cooking marjoram and oregano are interchangeable. If you're oregano-ed out, marjoram lends sweet pine and citrus notes to dishes.