Put a big pot of salt water on the fire, to use to get the bitterness
out of the fiddleheads.
Rinse and drain the scallops. The scallops need to be as dry as possible
and since they're underwater filter feeders, dryness is not their normal
state. Leave them in a colander while you work on the fiddleheads.
Rinse and trim the fiddleheads. Good fiddleheads should be tightly
coiled, not show too much leaf, not have any brown or black bits,
and since you're going to snap off whatever you can of the straight
stalk, there's no point in buying ones with really long stalks.
When the salt water comes to a boil, put the fiddleheads in and cook
them until they're not quite al dente. Then throw out the water
(which will be brown), and leave the fiddleheads covered in a colander,
so that they'll be warm and dry when you need them.
Mix together the corn starch and the garlic salt on a dinner plate.
I'm guessing it's a 3:1 ratio, but I'll measure it next time.
Take each scallop, squeeze it gently in paper towel to remove remaining
moisture, and roll it lightly on the plate.
You'll probably want to do this in two or three batches, so just do
the first batch for now.
Heat a frying pan medium-high, and add 2 T or so of butter.
When it melts, add the first batch of scallops.
There should be enough butter that the scallops look like they're
sitting in 1-2 mm (1/16 - 1/8 in) of it.
Now comes the trickiest part.
If all goes well, in a few minutes, each scallop will brown slightly
around the edges, and the butter near the scallop will brown too.
Flip the scallop over and you should see its bottom lightly browned.
Let it cook for another minute or two and then remove from heat to
a large, warm bowl.
If you're not sure if it's done, you can try cutting one open, but
just one.
It should look like it is just on the verge of losing its translucence
on the inside, and it'll lose it completely in a few minutes even when
removed from the heat.
You can also use the jiggle test: if you tap it with a fork in the pan
and it jiggles, then it's done.
If it's raw, the shock wave from the tap gets absorbed by the uncooked
middle.
That was if things go well.
What can go wrong?
If you're getting dark brown or black bits in your butter, the heat is
way too high. Fish them out if you can, and turn the heat down.
If you're getting a foamy mess in the pan, you didn't drain the
scallops well enough, or had the heat too low.
If you have enough uncooked scallops left, you can clean the pan and
start over, but otherwise you're stuck, so don't let this happen.
If the scallops are rubbery, you cooked them too long.
If they're squishy, you didn't cook them long enough.
Do the remaining scallops in batches, adding butter as needed as the old
butter boils off. At the end, put the fiddleheads into the butter, and
finish cooking them, adding about half the lemon juice to taste.
Mix the fiddleheads and scallops, either in the pan or the bowl,
add fresh black pepper and more lemon juice to taste.
Eat immediately.