Melt butter in large saucepan; cook and stir onions over medium heat about 10 minutes. Stir in water and broth mix. Bring to boil. Cover and simmer 15 minutes.

Smoothly combine flour and milk. Add to saucepan.

Cook and stir over medium heat until mixture boils and thickens. Remove from heat. Toss together Canadian Mozzarella and Swiss cheeses. Add 3/4 cup (175 mL) of the cheese mixture to the soup; stir until melted. Add salt and pepper to taste. Ladle into 4 oven-proof soup bowls; sprinkle with Buttered Croutons. Top with remaining cheese. Broil until cheese is melted. Serve.

Buttered croutons:
Toss 1 1/2 cups (375 mL) bread cubes in 3 tbsp (45 mL) melted butter. Bake in 350 ºF (180 ºC) oven 5 to 7 minutes. Turn cubes over; bake 5 to 7 minutes longer.

Makes about 1 cup (250 mL).