Heat oil in a large skillet over medium heat and sauté onions until caramelized. Mix farmer cheese, ricotta cheese, eggs, flour and salt in a large bowl. Immediately before frying, stir in mushrooms and caramelized onions.

Heat oil in a frying pan. Drop spoonfuls of batter into the pan and cook for approximately three minutes. Place a few mushroom slices on each latke, then flip. Cook for another three minutes or until cooked through.