Trim the fiddlehead ferns, removing any brown ends or mushy parts. Rinse them clean in cool water. Only do this right before cooking them—the added moisture will make these delicate fronds spoil if done too far ahead of time. If using the garlic, peel it and slice it very thinly.
In a large pot bring 2 quarts water to a boil. Add the salt and the cleaned fiddleheads. Cook for 1 minute. Drain and rinse with cold water until the fiddleheads cool off (or dunk them in a bowl of ice water to cool them). Drain them and lay them out on layers of paper towels to pat them dry. This process of blanching removes the bitter edge of fiddleheads. If their bitterness doesn't bother you, feel free to skip this step.
In a large frying pan, heat the oil over medium-high heat. Add the blanched fiddleheads. Cook, stirring frequently, until they start to brown on the edges, about 5 minutes. Add the garlic, if you like, and cook, stirring constantly, until the garlic is fragrant and just starting to color, about 1 minute
Salt to taste and serve immediately.
* Always look for fiddleheads that are green and bright, with minimal browning on the edges.