In a small bowl, combine the first 6 ingredients; spread half over 2 bread slices. Layer with chicken, bacon, mozzarella cheese, onion and tomato. Spread remaining mayonnaise mixture over remaining bread slices; place over top. Brush outsides of sandwiches with butter.
Cook in a panini maker or on an indoor grill until bread is browned and cheese is melted, 3-4 minutes.
Test Kitchen Tips
Go for freshly squeezed lemon juice when preparing this summery sandwich. Bottled lemon juice, which is from concentrate, won’t provide the same bright, fresh flavor. Buy some lemons to get the job done.
In Europe, Parmigiano-Reggiano and Parmesan are considered the same cheese. But in the U.S., Parmesan is a generic term that may not come from Italy’s Parmigiano-Reggiano region. Using the authentic Italian cheese (in a lesser amount than the original’s ½ cup) ensures a cheesy richness in the makeover with less fat and calories.