For the Seasoning Mix: Combine the coriander and dill seeds, red pepper flakes, garlic, salt, pepper, and paprika. Mix well and spread on a plate.
Using a vegetable peeler, remove just the yellow part of the peel (the zest) in long strips (reserve the lemon for the recipe or another use.) Twist and squeeze the strips of lemon zest over the spices to release the lemon oils.
Place the spices and the peels in a reclosable bag and shake well for half a minute. Scrape any spices off the peels and discard the peels. (See Cook's Note)
For the Broiled Cod: Set broiler to Low (or place rack in lower slot.) Mix together the butter and parsley.
Dry the fillets well and brush the butter mixture on both sides. Place the fillets on a broiling pan and sprinkle the tops with 2 tablespoons (more or less) of the spice mixture.
Broil fillets for 8-10 minutes per inch of thickness (measured at the thickest point) or until fish flakes readily. Sprinkle with fresh lemon juice and serve.
To store the Spice Mix: After removing the peels, spread the spices out on a plate to dry for 30 minutes. Store in an airtight jar.