Chipotle Chicken Tostadas

   Ingredients 

•  lbs. chicken breast
  
• 1/2 white onion, sliced
  
• 1 garlic clove, minced
  
• 4 plum tomatoes, diced
  
• 2 chipotle peppers La Costeña, chopped
  
• 1 can refried beans La Costeña
  
• 1 bag of tostadas
  
• Salt and pepper to taste
  
• Cooking oil as needed