In a skillet, heat olive oil over medium heat. Add diced onions and sauté for 3–4 minutes until soft. Stir in the garlic, cumin, chili powder, smoked paprika, salt, and pepper. Cook for 30 seconds until fragrant.
Add the tomato paste and stir well to combine with the onions and spices. Pour in the chicken broth and add the shredded chicken. Simmer for 5–7 minutes until the chicken is heated through and coated in the sauce. Set aside.
Spread a layer of refried beans onto each tostada shell. Top with a generous spoonful of the seasoned chicken. Then layer with shredded lettuce, tomatoes, cheese, cilantro, and your favorite toppings.
Finish with a dollop of sour cream, a splash of salsa or hot sauce, and a squeeze of lime. Serve immediately while the shells are crispy.