Step 1

Cook pasta to al dente according to package directions.

Step 2

Heat olive oil and butter over medium-low heat in a large pan. Add onions and cook for 15 minutes, stirring occasionally. Sprinkle with salt and sugar and cook another 15 minutes.

Step 3

Add garlic and cook 1 minute.

Step 4

Sprinkle flour over onions and stir to mix in. Turn heat up to medium. Cook 30 seconds and then gradually whisk in beef broth.

Step 5

Add sherry, onion soup mix, half-and-half, and thyme and simmer until thickened.

Step 6

Stir in chicken and cooked pasta.

Step 7

Transfer to a lightly greased casserole dish. (If your pan is oven-proof, there's no need to transfer. Just sprinkle the cheese on top.) Sprinkle both cheeses on top.

Step 8

Place casserole dish (or pan) in oven and broil until the cheese is melted and golden brown in spots.