How to Make It
Preheat the oven to 350°. Toast the walnuts for about 7 minutes, or until golden; let cool. In a food processor, combine the sun-dried tomatoes, Parmesan, basil, garlic and walnuts and pulse until finely chopped. Add the 2 tablespoons of oil and process until smooth. Season the pesto with salt and pepper.
Light a grill or preheat the broiler. Brush the bell peppers, zucchini and eggplant with olive oil, season with salt and pepper and grill over a medium-hot fire or broil for 12 to 15 minutes, turning, until lightly charred all over and very tender. Transfer the bell peppers to a bowl, cover with plastic wrap and let cool. Peel, core and seed the bell peppers and cut into quarters.
Slice open the baguette and discard the center, leaving a scant 1/2 inch of bread all around. Spread a thin layer of pesto on both sides of the baguette and arrange half of the spinach on the bottom. Top with half of the cheese and vegetables, then repeat with the remaining ingredients. Cut the muffuletta in half. Tightly wrap the sandwich halves in plastic and let stand at room temperature for 4 hours or refrigerate overnight. Cut each sandwich into 4 pieces and serve.