How to Make It
Remove top oven rack, and cover with foil; lightly coat with cooking spray. Preheat broiler to high.
Arrange tomato slices on prepared rack; lightly coat with cooking spray. Broil 8 to 10 minutes or until tomatoes are lightly browned. Transfer tomatoes to a plate; set aside.
Combine flour and butter in a bowl until a paste forms.
Place a large high-sided sauté pan over medium-high heat. Add stock, 2 cups milk, salt, and mustard, stirring with a whisk. Add pasta to pan; bring to a boil. Cook 12 minutes or until pasta is done, stirring frequently. Stir in remaining 1 cup milk and artichokes; cook 2 minutes. Add butter-flour paste, in pieces, stirring constantly to blend and thicken. Remove from heat; stir in cheddar, fontina, 6 tablespoons Parmesan, and pepper.
Arrange broiled tomato slices on top of pasta. Sprinkle with remaining 4 tablespoons Parmesan. Broil 2 minutes or until cheese begins to brown.