Preheat your oven to 400 F. Lightly grease a 9x13” casserole dish and line a baking sheet with aluminum foil.
Cut your squash in half, place cut side down onto the baking sheet and bake for 45 minutes- 1 hour, or until the squash flesh shreds easily with a fork. Allow to cool and reduce the heat to 375 F.
Meanwhile, heat the olive oil in a large nonstick skillet over medium heat. Add the onion, celery and carrot and sauté until the onion becomes translucent and the carrot softens, about 7 minutes.
Stir in the flour and cook on medium heat for 2-3 minutes until it fully coats the vegetables.
Whisk in the stock and milk, and bring the mixture to a simmer. Cook for 5 minutes until the mixture begins to thicken, then add in the thyme, parsley, mushrooms, broccoli, peas, tuna and squash. Stir until fully mixed then transfer to the casserole dish.
Top with the cheese and bake for 10 minutes. Remove the casserole and top with the chips and bake for an additional 10 minutes or until the casserole has thickened. Enjoy!