Gruyere, Asparagus & Pea Baked Pasta

Preheat oven to 375 degrees F. Coat a 9-by-13-inch baking dish with cooking spray.

Bring a large pot of water to a boil. Cook pasta for 2 minutes less than the package directions. Reserve 1/2 cup of the cooking water, then drain and rinse the pasta. Transfer to a large bowl.

Meanwhile, heat 1 tablespoon oil a large skillet over medium heat. Add leeks; cook, stirring occasionally, until very tender, 6 to 8 minutes. Add asparagus and garlic; cook, stirring, until the asparagus is tender-crisp, 2 to 4 minutes. Add peas; cook 1 minute more. Transfer to the bowl with the pasta.

Add 2 tablespoons oil to the pan and return to medium heat. Sprinkle in flour; cook, whisking, until it starts to brown, 2 to 3 minutes. Whisking constantly, slowly pour in milk, then the reserved water, mustard, salt and pepper. Bring to a simmer and cook, whisking, until the sauce thickens, 2 to 3 minutes. Remove from heat; whisk in cheese and tarragon. Pour over the pasta mixture and stir until coated. Transfer to the prepared baking dish.

Combine breadcrumbs and the remaining 2 tablespoons oil in a small bowl and sprinkle over the casserole. Bake until steaming hot, 30 to 35 minutes. Let stand 10 minutes before serving.