Trim and rinse fiddleheads, removing any brown ends or mushy parts.
In a large pot bring 2 quarts salted water to a boil. Add fiddleheads and cook 1 minute. Drain and rinse with cold water.
In a large frying pan, heat oil over medium-high heat. Saute onion for 3 minutes.
Add fiddleheads. Cook, stirring, until they start to brown, about 5 minutes. Add tomatoes, garlic, parsley and red pepper flakes, stirring, until garlic is fragrant and just starting to color, about 1 minute. Salt or squeeze lemon to taste. Serve immediately.