Slice the yellow squash and zucchini length wise so that you have long slices about a 1/4 inch thick. Slice the eggplant into 1/2 inch circles and then cut in half. Cut the red pepper into 1 inch strips. Place all the vegetables in a large plastic bag.
Make the marinade by mixing together the sea salt, Italian seasoning, pepper, minced garlic and olive oil. Stir all together in a small bowl. Pour over the vegetables. Seal up the bag and gently massage the vegetables so they are all coated with the seasoning. Allow to sit for 15 minutes.
Meanwhile heat a grill pan over high heat. Place the vegetables in a single layer on the grill pan. It took me about 3 batches of vegetables to get them all grilled. You want to grill the vegetables so that they are cooked all the way through and just starting to get soft. The times definitely vary by vegetable. Here are the total cooking times for each, flip halfway through. -Red peppers: 25 minutes-Zucchini/Yellow squash: 8-10 minutes-Eggplant: 5-6 minutes Remove vegetables from grill pan and allow to cool for a minute or two. Lower grill pan temperature to medium. Slice the focaccia into thirds. Take one of the pieces and slice in half. Spread pesto on both sides of the bread. Layer 2 slices of yellow squash, 3 slices eggplant, 2-3 slices eggplant, and 3-4 slices of red pepper on the focaccia. You should have enough veggies to evenly divide amongst the three sandwiches. Top the veggies with 3 slices fresh mozzarella. Cover with the top slice of focaccia.
Lightly spray the grill pan with cooking spray. Place the sandwich on grill pan and cook 4-5 minutes per side so that the bread is toasted and golden brown. Using the lid of a pan press down on the sandwich to get the classic panini pressed look without a panini press. If you have one you can definitely cook the sandwich in that. Flip the sandwich over and press down again.
Remove panini from pan, cut in half and enjoy. You can keep the extra ingredients in the refrigerator for a few days and then are ready to make your next panini.