Fit a large pot with a steamer basket, and fill it with water so that it reaches the same level as the steamer. If you don’t have a steamer, fill the pot with 5-6 cups of water - you will boil the fiddleheads instead.
Bring to a boil over high heat, remove the steamer basket (if using), and add a generous amount of salt.
Add the fiddleheads to the steamer basket, or directly to the boiling water. Steam for 12 minutes, or boil for 15 minutes, until the stems are tender, but still bright in color.
Drain the fiddleheads in a colander over the sink.
Heat a medium-sized skillet over medium heat and add the butter, stirring to melt.
Add the fiddleheads into the simmering butter, and toss to coat.
Saute for 2-3 minutes, or until the edges begin to brown and everything is coated in melted butter.
Squeeze in the lemon juice and add kosher salt and black pepper to your taste.
Serve immediately.