Slice zucchini lengthwise in 1/4" strips.  Saute' zucchini and mushrooms in a pan over medium heat for a couple minutes or until slightly softened.

Slice tomato and onion. Shred mozzarella and combine with feta and basil.

Assemble paninis on an unheated griddle. Layer one slice of bread with a small amount of the cheese mixture,  half the veggies (including the spinach and peppers), another thin layer of cheese, the remaining veggies, and one more layer of cheese.

Top with a slice of bread and heat over medium heat until browned on both sides and cheese is melted.