How to Make It
Preheat the broiler and position a rack 8 inches from the heat. Put a kettle of water on to boil. In a large, deep ovenproof skillet, melt the butter. Add the onion and cook over moderate heat, stirring frequently, until softened, 5 minutes. Add the tomato paste and thyme and cook for 1 minute. Whisk in the half-and-half and 2 cups of hot water and bring to a simmer. Season with salt and pepper. Add the macaroni and cook over moderately low heat, stirring frequently, until the pasta is al dente, about 8 minutes. Stir in 1/4 cup of boiling water along with the Gruyère, cheddar and Fontina. Cover and let stand off the heat for 2 minutes, until the cheese is melted. Season with pepper and stir once or twice.
In a bowl, combine the Parmigiano and panko and sprinkle it over the pasta. Broil for 2 minutes or until golden. Serve.