Chicken Fajita Soup

1 pound boneless, skinless chicken breasts

1 medium onion                               

1 teaspoon butter

4 cups chicken broth                       

2 cups beef broth                              

1 (14.5-ounce) can diced tomatoes

1/4  cup chopped green bell pepper
1 to 2 tablespoons chili powder
1/2 teaspoon ground cumin
Salt and ground black pepper 

Tortilla chips
Grated Cheddar cheese
Avocado slices
Lime wedges

Cut the chicken into bite-size pieces. Chop the onion. Melt the butter in a large pot. Cook and stir the chicken and onion over medium-high heat until the onion is soft and the chicken is no longer pink. Stir in the chicken broth, beef broth, tomatoes, bell pepper, chili powder, and cumin. Bring to a boil, then decrease the heat and simmer for 20 minutes. Season with salt and black pepper to taste. Serve in bowls and top with the tortilla chips, cheese, and avocado slices. Pass the lime wedges. Makes about 10 cups