Preheat oven to 375°F. Cook noodles to al dente and drain.

Steam broccoli until tender, about 4 minutes; drain well.

Melt butter in a saucepan over medium heat. Whisk in flour for 1 minute.

Slowly add milk, whisking until thickened, about 3–4 minutes.

Remove from heat and stir in cheddar, salt, and pepper until smooth.

Fold noodles, broccoli, and tuna into the cheese sauce. Pour into a 9x13-inch dish.

Sprinkle breadcrumbs over the top and bake 15–18 minutes until bubbly and lightly browned.