Mix mayonnaise and lemon juice in a bowl. Add tuna and blend, then season to taste. If desired, add lemon pepper or chopped herbs to taste.

Slice each piece of bread into two (from side). Spray outside of bread with cooking spray and lay sprayed side down on cutting board. Spread bottom half of bread with tuna mixture, top with some fresh spinach and add a slice of cheese. Top with other half of bread.

Preheat a panino grill, or heat a grill pan over medium heat. Spray with cooking spray to prevent sticking. Grill sandwich until the cheese melts and the bread is crisp and golden, about 5 minutes. If using a grill pan, press down on top of sandwich while grilling to compact (I used a cast iron skillet) and you will need to turn sandwich over halfway through to achieve that lovely pressed, grilled texture on both sides. Try not to squish all the goodies out the side like I did.

Serve immediately.