Heat vegetable oil in a medium fry pan. Saute onion for 1 to 2 minutes.
Add sausage. Break it up with your spatula and continue to fry for 4 to 5 minutes until it is fully cooked. Transfer to a bowl.
Add sliced black olives and chopped cilantro. Mix well with the cooked onion and sausage.
Preheat oven to 350˚F (180˚C).
Place mini fillo shells on baking tray. You may need to use two baking trays.
Place about ¾ teaspoon sour cream at the base of the fillo shells. Then add 1 to 2 teaspoons of sausage filling. Finally, top with shredded mozzarella.
Bake in preheated oven for about 10 minutes. Remove and transfer to a serving dish. Sprinkle some chopped cilantro over the top. Serve immediately.