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.