Slice potatoes into 24 (1/4-inch-thick) rounds. In a vegetable steamer, set over boiling water steam 1 layer of potato slices, covered, until tender, 5 to 10 minutes, and cool completely. Steam remaining slices in same manner. Potato slices may be steamed 1 day ahead and kept in a sealed plastic bag, chilled. Bring potato slices to room temperature before proceeding with recipe.
To make "tartare": In a bowl stir together "tartare" ingredients and salt and pepper, to taste.
To assemble hors d'oeuvres: Brush tops of potato slices with lemon juice and season with salt and pepper. Top each slice with a heaping 1/2 teaspoon "tartare" and a small dollop sour cream.