Beat the egg yolks with salt and pepper in a shallow pan (a pie pan works great) to make an egg wash.
Combine the potato flakes with the thyme leaves and spread out on a plate or shallow pan.
Season the fillets with salt and pepper on one side only. Dip just the seasoned side of the fish into the egg wash, being careful not to let any egg wash drip down the sides of the fish. Dip the egg-washed side of the fish into the seasoned potato flakes and press down a bit to make them adhere. Set aside, crust-side down. Repeat with the remaining fillets.
Pour olive oil into a large sauté pan, arrange the fillets crust side down in the pan, and place on the burner over medium-high heat. (Do not preheat the sauté pan; the fish will not stick.) After about 3 minutes, the crust should be golden brown; if it is not, raise the heat to high and continue cooking. When the crust has turned golden brown, season the fish with salt and pepper, flip it over, and cook for 1 minute. Remove from heat; leave the fillets in the pan for 1 minute.
Deglaze the pan with the lemon juice, being careful not to squeeze it directly on the fish, which would soften the crisp crust you have worked so hard to develop. Swirl the pan a few times to incorporate the lemon juice. Remove from pan and serve!