Copycat Recipe: Panera Bread French Onion Soup

I tend to double this recipe because it is so great, and makes absolutely wonderful leftovers!

1/4 cup butter
8 cups sliced yellow onions (4 to 5 medium onions)
2 14oz cans beef broth
1 cup chicken broth
3 Tbsp flour or cornstarch
1 Tbsp ketchup
1 1/2 tsp ground black pepper
1 1/2 tsp salt
1/4 tsp dried thyme
1/2 tsp minced garlic

Melt butter in saucepan over medium heat. When butter is melted, add onions and saute, stirring often, until onions turn brown.  This takes quite a while to achieve the desired caramelization; the more caramelized, the better your soup will be! Add remaining ingredients and stir to combine. Heat soup until it's boiling, then reduce heat and simmer, uncovered, for 20 minutes. Garnish with croutons and shaved asiago cheese.  Makes approximately 7-8 cups.