1. Preheat oven to 375°F and lightly coat a 9×13-inch baking dish with non-stick spray.

2. Cook noodles according to package instructions in a large pot of salted water until very al dente. Drain, and transfer noodles back to pot (removed from burner).

3. Heat oil in a large sauté pan over medium-high. Once hot, add mushrooms and onion; cook until softened, about 8 minutes. Stir in garlic and fresh thyme; cook until fragrant, about 1 more minute.

4. Add butter to pan; stir to melt. Sprinkle flour over mixture, and stir well to coat. Cook 2 full minutes, stirring often, to eliminate raw flour taste. Whisking constantly, gradually pour in vegetable broth; bring to a boil.

5. Reduce to a simmer and gradually whisk in milk, Dijon mustard, salt, pepper, and celery seed. Bring mixture back to a simmer; simmer 3 minutes, or until slightly thickened. Sprinkle in half of the cheese and stir constantly until melted. Add lemon juice and zest.

6. Pour milk mixture into pot with cooked noodles; add tuna and green peas. Stir well until all components are well-combined. Transfer mixture to prepared baking dish; sprinkle remaining cheese and breadcrumbs overtop.

7. Bake casserole at 375°F until bubbly and cheese is melted, about 15 minutes. For a crisper, golden topping, broil for final 2 minutes. Let stand at room temperature 5 to 10 minutes before digging in. Garnish with additional fresh thyme, if desired.