Have ingredients at room temperature. Make Lemon Aioli (flavored mayonnaise) by whisking together the lemon zest, juice and garlic, which has been slightly mashed with the salt (use a sharp knife tip on a cutting board). Whisk in the mayonnaise and olive oil. Spread on smooth side of each square of focaccia, using a generous 1/2 tablespoon.
Divide remaining ingredients among 4 squares focaccia, layered in the order listed. Top with remaining 4 squares focaccia. Cut each sandwich in half into two triangles.
Sandwiches can be served at room temperature or warm in a 450-degree oven a few minutes, especially if regular mozzarella was substituted for fresh. Sandwiches are not meant to be served hot.