Fiddleheads are edible ferns that grow wild along the fertile banks of rivers and streams. You may find them with a rather unattractive brown, somewhat papery residue on them, which should simply be washed off before preparing them.
In a large pot of boiling, salted water, cook the linguine until al dente. Drain and transfer to a large bowl.
Meanwhile, remove the papery particles from the fiddleheads. Fill a medium bowl with cool water; add 1 teaspoon salt and the lemon juice. Add the fiddleheads, and push them down into the water several times to clean them. Transfer them to a steamer rack in a saucepan and steam, covered, about 4 to 5 minutes.
Heat the olive oil in a large skillet over medium heat. Add the leeks and saute for 2 to 3 minutes, until soft. Add the fiddleheads and cook 1 to 2 minutes more, until warm and golden. Stir in the dandelion greens. Toss the mixture with the pasta, season with salt and pepper, and serve.