How to Make It
Heat oven to 375° F. Butter a 9-by-13-inch baking dish. Cook the noodles according to the package directions, adding the asparagus and peas 2 minutes before the noodles are done. Drain and return to the pot.
Melt the butter in a large saucepan over medium heat. Transfer 2 tablespoons of the butter to a small bowl, add the bread crumbs and ¼ teaspoon each salt and pepper, and toss to combine; set aside.
Cook the onion in the remaining butter in the saucepan, stirring often, until softened, 3 to 5 minutes. Add the flour and cook, whisking, for 1 minute (do not let darken). Gradually whisk in the milk; season with 1½ teaspoons salt and ¼ teaspoon pepper. Bring to a boil, reduce heat, and simmer, whisking occasionally, until thickened, 5 to 7 minutes.
Remove the flour sauce from heat, add the Cheddar, and stir until melted. Add to the drained pasta, along with the tuna and tarragon; toss gently to combine. Transfer the mixture to the prepared baking dish and sprinkle with the bread crumbs. Bake until browned and bubbly, 30 to 40 minutes. Let cool for 5 minutes before serving.