Begin by cooking the spinach according to the package directions, omitting any salt. Drain well, and then place in a 12x8x2-inch casserole dish. Sprinkle with lemon juice.
Rinse the fillets well in cold water. Pat them dry with paper towels and then arrange on the spinach.
In a small saucepan over low heat, melt the margarine and then add the flour, stirring until smooth. Stirring constantly, cook for 1 minute and then gradually add the milk.
Continue to cook over medium heat, stirring constantly, until the mixture has thickened and has become bubbly.
Remove the pan from the heat, and stir in the egg yolk. Add the shredded Swiss cheese, and stirring frequently, cook over low heat until the cheese melts.
Preheat your oven to 350F and then spoon the sauce over the fish. Sprinkle each fillet with Parmesan cheese and then bake for roughly 20 minutes, or until the fish flakes easily when tested with a fork.