01 - Preheat the oven to 390°F. Lightly grease a medium baking dish with cooking spray or oil.
02 - Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Drain well and set aside.
03 - Heat 1 tbsp 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 tbsp olive oil in a saucepan over medium heat. Stir in flour and cook 1 minute, stirring constantly. Gradually whisk in lactose-free milk until smooth. Simmer 3-4 minutes, stirring constantly, until thickened. Whisk in mustard, oregano, basil, salt, and pepper.
05 - In a large bowl, combine cooked pasta, sautéed vegetables, drained tuna, and prepared sauce. Fold gently until evenly coated.
06 - Transfer mixture to prepared baking dish. Sprinkle evenly with lactose-free grated cheese and breadcrumbs if using.
07 - Bake for 20 minutes until cheese is melted and golden, and edges are bubbling. Let rest 5 minutes before serving.