Step 1

Preheat the oven to 425 degrees F. Generously spray the bottom and sides of a 9-by-13-inch casserole dish with nonstick spray.

Step 2

Whisk the chicken broth, heavy cream, sour cream, cream cheese, mustard, scallions, 1 1/2 teaspoons salt and a few grinds of pepper in a large bowl until smooth and no lumps of cream cheese remain.

Step 3

Spread half the noodles in an even layer in the bottom of the prepared dish. Scatter half of the tuna, mushrooms and peas over the pasta. Pour half of the broth mixture over the top, then sprinkle with 1 cup of the Cheddar. Repeat to make a second layer with the remaining noodles, tuna, mushrooms and peas.

Step 4

Cover tightly with foil and bake until the noodles are al dente, about 40 minutes. Uncover and sprinkle the remaining 1 cup Cheddar evenly on top. Bake until the cheese has melted and the sauce has thickened slightly, 10 to 12 minutes. Let the casserole sit for about 10 minutes before serving.