1. To caramelize onions, preheat a skillet over medium-high heat. Add olive oil to the pan and the thinly sliced onion and toss to coat. Let simmer and stir occasionally until the onions turn golden brown and reduce down. Remove from the skillet.2. Slice zucchini and red peppers into strips.3. For the paninis, preheat a panini maker to 375ºF. (If you don’t have a panini press, make this like a grilled cheese—it’s fantastic!)4. Top slices of bread with mayonnaise and mustard. Top slice of bread with turkey, tomato, caramelized onion, spinach, red pepper and then cheese. Top with another slice of bread.5. Spray panini press with nonstick spray and press until bread is golden brown and the cheese has melted inside. Remove from the press, slice and serve.