Chipotle Chicken Tostadas

Ingredients

• 2 tablespoons vegetable oil
• 2 medium onions, halved, thinly sliced
• 2 pounds skinless boneless chicken thighs
• 1 3/4 cups chicken stock or canned low-salt chicken broth
• 2 cups chopped tomatoes
• 6 cloves garlic, minced
• 3 teaspoons minced canned chipotle chilies
• 12 purchased tostada shells
• 8 ounces iceberg lettuce, shredded
• 12 tablespoons sour cream
• 1 cup queso fresco or feta cheese
• 4 radishes, halved, thinly sliced