Preheat oven to 400°F.
Spread a thick layer blackening spice evenly on a plate.
Lay 1 salmon filet onto the spice, pressing lightly to coat as completely as possible. Turn over and repeat on second side. Place on a large plate.
Repeat with remaining filets. Lay in a single layer on plate, do not stack.
Place a 12-inch cast iron skillet over medium high heat.
When skillet it is very hot, place 2 salmon filets in the pan, do not crowd or stack.
Drizzle oil over tops and around edges of the fish.
Sear for about 1 1/2 minutes, until golden brown and not burned. Turn and sear for 2 minutes more.
Transfer pan to preheated oven. Cook for 7 minutes, without turning.
Transfer a salmon fillet to each plate, set aside and keep warm.
Repeat with last 2 filets. (Naturally, all four filets may be cooked at once if you have a larger cast iron skillet or 2 cast iron skillets.)
Place chopped parsley on a plate.
Dip both sides of each lemon slice into the chopped parsley. Place 1 on the side of each fillet.
Scatter a little more chopped parsley around the edge of each plate.
Serve immediately.