01 - Preheat the oven to 390°F. Lightly grease a medium baking dish with cooking spray or a small amount of oil.
02 - Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to package directions. Drain thoroughly and set aside.
03 - Heat 1 tbsp olive oil in a skillet over medium heat. Add the onion, zucchini, and bell pepper. Sauté for 5 minutes until softened. Add cherry tomatoes and baby spinach, cook for another 2-3 minutes until spinach wilts. Remove from heat.
04 - Heat remaining 1 tbsp olive oil in a saucepan over medium heat. Stir in the flour and cook for 1 minute, stirring constantly. Gradually whisk in the lactose-free milk until smooth. Simmer for 3-4 minutes, stirring frequently, until thickened. Stir in mustard, oregano, basil, salt, and pepper.
05 - In a large mixing bowl, combine the cooked pasta, sautéed vegetables, drained tuna, and prepared sauce. Gently fold together until evenly coated.
06 - Transfer the mixture to the prepared baking dish, spreading evenly. Sprinkle lactose-free cheese and breadcrumbs (if using) over the top.
07 - Bake for 20 minutes, or until the top is golden brown and the sauce is bubbling around the edges. Allow to rest for 5 minutes before serving to set.