Heat a drizzle of olive oil in a frypan over medium heat. Sauté onions until translucent then add the garlic, lentils, spinach and mushrooms. Cook for 6-7 minutes or until excess water has evaporated. Take off heat, and cool.
Add the feta and egg to the cooled vegetables. Mix to combine.
Taking 2 sheets of filo at a time, brush both sheets with olive oil.
Cut sheets into 4, lengthwise. Place a spoonful of filling at one end and fold the corner over diagonally. Continue folding until you reach the end of the filo maintaining the triangle shape. Repeat with the remaining filo and lentil filling.
Bake for 15-20 minutes or until triangles are crispy and golden.