Preheat oven to 375 and start pasta water.
In a large saute pan, melt 2T of butter over low heat. When butter has melted stir in the bread cubes and sprinkle on a large pinch of salt. Turn up heat to medium and stir until bread is slightly toasted. Set cubes aside.
Cook pasta until al-dente, strain well and set aside in a large bowl
Place the remaining 4T in the saute pan and melt over medium heat. When butter is melted whisk in flour. Keep whisking for about 4 minutes to cook out the raw taste from the flour. Whisk in milk and whisk for 5 more minutes.
Whisk in tomato paste and 1.5c of each kind of cheese.
When cheese has all melted into the sauce, pour the sauce into the bowl with the pasta and mix well. Pour the pasta and sauce in a large rectangular baking dish, spread out the bread on top and put the remaining 1/2c of each cheese on top.
Bake in the oven about 40 min until the cheeses are bubbly. Cover with foil if the top gets too dark. Let rest 10-15 minutes before serving or cover and reheat the next day
(from Williams Sonoma catalog)