Melt butter over medium-low heat in a large pan.
Sprinkle flour and stir to mix in. Turn heat up to medium.
Cook 30 seconds and then gradually whisk in french onion soup, half-and-half and thyme and simmer until thickened.
Stir in chicken and cooked pasta, transfer to a lightly greased casserole dish.
Sprinkle both cheeses on top.
Place casserole dish in oven and broil until the cheese is melted and golden brown in spots.