1) In a medium bowl, mix together the mayonnaise and the lemon juice. Add the tuna, salt, and pepper, and stir to combine.
2) Cut the focaccia in half and brush the outside of the bread with olive oil. Place a slice of cheese on the bottom half of the panino. Top with tuna mixture, then the frisee, and then another slice of cheese. Finish with the top half of the focaccia.
3) Preheat a panino grill, or heat a griddle pan over medium heat. Drizzle with olive oil to prevent sticking. Grill the panini until the cheese melts and the bread is crisp and golden, about 5 minutes. Cut the panino in half and serve.