Step 1

Combine pasta, chicken, tomatoes and 1/4 cup basil in a large ovenproof skillet. Whisk broth, milk, sour cream, flour, onion powder, dry mustard, salt and pepper together in a measuring cup. Pour over the pasta mixture. Bring to a simmer over medium-high heat. Reduce heat to maintain a simmer, cover and cook, stirring twice, until the pasta is tender and the chicken is cooked through, 14 to 16 minutes.

Step 2

Meanwhile, position rack in upper third of oven; preheat broiler to high. When the casserole is done, sprinkle cheese on top and broil until the cheese is melted and lightly browned, 2 to 3 minutes. Serve sprinkled with the remaining 1/4 cup basil and a drizzle of vinegar, if desired.