Toast the croutons. Scatter the rye bread cubes from 2 slices rye bread in a baking dish and toast at 370 °F for about 15 minutes, until crisp. Set aside to cool.
Heat the beer. Bring 4 ¼ cups beer to a boil in a pot, watching so it does not boil over. Lower the heat and skim off the foam with a slotted spoon.
Make the cream thickening (záklechtka). Melt 2 Tablespoons unsalted butter in a small saucepan, then remove from the heat. Pour in ¾ cup heavy cream, sprinkle in 3 Tablespoons all-purpose flour, and stir smooth. Whisk in 5 egg yolks, then 1 ½ Tablespoons granulated sugar, 1 ½ teaspoon salt, and ¼ teaspoon mace, until no lumps remain.
Combine and thicken. Warm the skimmed beer over low heat and, stirring constantly, pour in the cream mixture. Cook very gently for about 10 minutes, stirring often, until thickened. Do not let it boil hard, or the yolks will curdle.
Finish and season. Off the heat, stir in 2 Tablespoons lemon juice. Taste and balance with more salt, sugar, or lemon. Serve topped with the croutons, parsley, and a little ground pepper.