Spray a nonstick skillet with cooking spray, set it over medium heat and allow it to get hot. Add onions and sautee about 2 minutes, or until tender.
Add peas & carrots, tuna and soup. Do not dilute soup. Heat an additional 2-3 minutes, stirring to combine veggie mixture with soup.
Add zoodles and stir until completely integrated. Transfer to a very small casserole dish or into individual ramekins.
In the microwave, melt butter then mix in bread crumbs and Parmesan cheese. Stir until thoroughly combined. Top casserole with crumb mixture and bake at 350 degrees for 30 minutes or until topping is crispy.