Preheat the oven to 350 degrees F (175 degrees C).
Bring a large pot of lightly salted water to a boil. Cook fusilli in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes.
Meanwhile, melt butter in a saucepan over medium heat. Whisk in flour, stirring constantly for about 1 minute. Remove from heat and gradually pour in milk, whisking constantly the entire time to avoid lumps from forming. Return to heat and cook, stirring constantly, until slightly thickened, about 2 minutes. Stir in 1/2 of the Cheddar cheese. Add lemon juice, mustard powder, salt, and pepper and mix well.
Drain fusilli and fold into the sauce. Mix in tuna. Pour mixture into an 8-inch casserole dish and sprinkle with breadcrumbs. Top with remaining Cheddar cheese.
Bake in the preheated oven until cheese is melted and golden, about 30 minutes.