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