Sautee the onions and peppers for about 5 minutes. Add the beans, corn, and spices. Salt and pepper to taste.
To make the guacamole, mix the mashed avocado, sour cream, onions, lime, and salt.
To make the sour cream mixture, mix sour cream, onions, cucumbers and cumin.
Spread the guacamole on top of the prepared tostada shell. Layer the cooked salmon on top, spooning the bean mixture next. Add a small amount of sour cream mixture, ending with cheese, tomatoes, and fresh cilantro. Serve!