Wash fiddleheads and set to boil in water for 15 minutes. Fry 1 cm cubes of paneer cheese or extra firm tofu in olive oil in a large frying pan. Flip when one side is browned, then add sliced mushroom, shallot (cut into eighths vertically, not too small), salt and pepper. Once fiddleheads are soft, drain, and add to pan with other ingredients. Stir-fry for several minutes, then serve topped with lemon juice.