How to Make It
Preheat oven to 375°F. Cook noodles according to package instructions until very al dente, omitting salt and fat. Drain and transfer to a large bowl.
Heat oil in a large high-sided skillet over medium. Add mushrooms; cook until softened, about 4 minutes. Add onion and garlic; cook until tender, about 4 more minutes. Add thyme; cook until fragrant, about 1 minute, stirring constantly.
Add butter; stir to melt. Whisk in flour; cook until roux is golden, about 1 minute. Whisking constantly, add stock; bring to a boil. Reduce to a simmer; whisk in milk, tomato paste, mustard, lemon zest and juice, salt, and pepper. Return to a simmer; simmer 2 to 3 minutes. Pour mixture into bowl with noodles. Stir in tuna and dill.
Coat a 13- x 9-inch baking dish with cooking spray. Transfer tuna mixture to baking dish; top with cheese. Bake at 375°F until bubbly and cheese is melted, about 15 minutes. Let stand at room temperature 5 to 10 minutes. Garnish with dill.