In a large heavy bottomed skillet over medium heat, add olive oil and potato. Cook stirring occasionally for about 5 minutes.
Add onion to the skillet and continue cooking until the potato is cooked through and golden brown, about 10-15 more minutes.
In a large bowl combine cooked potato mixture with remaining ingredients. Divide mixture into 3 portions.
Working with one portion at a time, shape 4 fish cakes into 2 inch patties. Place them in a hot skillet and cook until golden brown on each side, and heated through, about 6 minutes.
Repeat with remaining portions. Makes 12 smoked trout fish cakes. Serve with mayonnaise.