For the arugula walnut pesto: Combine the Parmesan, walnuts, garlic and lemon zest in a food processor until smooth. Add the arugula, oil, basil amd lemon juice and sprinkle with salt and pepper and process until smooth. Add water as needed to smooth the consistency.
For the avocado melt: Smear the arugula walnut pesto on 1 side of each bread slice and top with the cheese. Toast the bread slices cheese-side on a hot grill until the cheese melts (tenting the bread helps to steam and cook faster). Top the slices with the avocado and bacon, if using. Close the sandwich and enjoy!
Cook's Note: The pesto keeps for up to 1 week in the refrigerator.
This recipe was provided by a chef, restaurant or culinary professional and may have been scaled down from a bulk recipe. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.