Preheat your oven to 400 F and line a baking sheet with parchment. Slice the zucchini in half and using a teaspoon, scoop out the center, about an inch. Spray each half lightly with cooking spray and roast for 15-20 minutes (this will depend on the size of your zucchini).
The key is to have them tender, but not mushy. Use a fork to see how easily you can pierce the side, look for slight browning on the side. Meanwhile, mix together the Genova tuna, celery, shallot, dill and parsley, and the lemon juice.
Once the zucchini is done remove from oven and turn on the broiler. Fill zucchini with the tuna mixture, sprinkle with chopped tomato and top with the Parmesan cheese. Broil for 2-3 minutes or until the cheese has browned and is crispy on top.