4 thin slices baguette or other artisan bread
1 tablespoon butter
1 large onion, thinly sliced
1 carton (32 oz) beef or chicken broth (4 cups)
1/2 teaspoon balsamic vinegar
Salt and pepper to taste
2 oz Gruyère, Swiss or fontina cheese, shredded (1/2 cup)
Heat oven to 400°F. Place bread slices on cookie sheet. Bake 10 minutes or until toasted.
Meanwhile, in 2-qt. saucepan, melt butter over medium heat. Cook onion in butter 20 to 25 minutes, stirring occasionally, until completely tender and starting to caramelize. Stir in broth, vinegar, salt and pepper.
Divide soup between 2 CORNINGWARE® COLOURS® POP-INS® 20-OZ MUGS. Top each mug with 2 slices of toast; sprinkle evenly with cheese.
Place mugs on cookie sheet. Bake 10 minutes or until cheese is melted and bubbly.