Cook the pasta according to the package instructions. Drain and set aside.
Heat half the butter in a braiser over medium heat. Add the chicken, then sprinkle with a pinch of salt and pepper. Cook until the chicken is browned on all sides and cooked through, about 4 minutes. Add the garlic, then stir and cook briefly. Add the chicken stock, scraping the bottom of the pan as it boils. Add the milk, cream and the remaining 3 tablespoons butter and cook, stirring, until it is warmed through and the sauce is simmering.
Toss together the Cheddar, fontina and cornstarch in a bowl.
Sprinkle the shredded cheese mixture into the pan, stirring constantly until the cheese is completely melted. Add the processed cheese cubes and allow them to melt. Bring to a simmer and allow the sauce to thicken, about 1 minute. Add the cooked macaroni, the spinach and tomatoes, then stir until well combined. Taste and adjust the seasonings. Garnish with the fresh parsley and serve.