Use cast-iron pan, 10-12 inches in diameter and 2-3 inches deep; OR any frying or saute pan deep enough to hold the egg mixture.
Parboil potatoes by placing them in a saucepan full of lightly salted water; bring to a boil, decrease heat to low and simmer until tender, about 15-20 minutes; drain.
Heat some olive oil in the pan, and saute the potatoes and onions until lightly browned, stirring frequently.
Season the potatoes and onions with salt and pepper; remove from pan and set aside.
Add a little more oil to the pan and saute the shrimp, lobster, and crabmeat in small batches until just barely cooked through; lightly salt, pepper, and add cumin to the seafood as you cook it, turning frequently (don't overcook; it is better to undercook the seafood a bit at this point); remove seafood from pan and set aside.
There should now be some oil remaining in the pan; for the next step you'll want just enough oil to cover the bottom of the pan; so if there is more than that, drain some off now.
In a bowl, beat the eggs with a wire whisk; add softened butter, whisking vigorously; you will now have little flecks of butter in the egg mixture.
Whisk in a pinch of salt and a dash of pepper; add potatoes, onions, and seafood to beaten eggs.
Heat the remaining oil in the pan to med-hot; pour the egg mixture in the pan and cover.
Immediately decrease heat to low; cook for approximately 7-10 minutes until the eggs are set and firm.
Cover the pan with a large plate and flip.
Return to the pan and fry the other side until just lightly browned.
Cut the tortilla into 12 wedges and garnish with chopped parsley.