This traditional tapa is served throughout every region of Spain, with little variation. A poorly made version will taste like a mouthful of mayonnaise. On the other hand, a well-made ensaladilla rusa is a perfectly balanced mixture of potatoes, hard-cooked eggs, and vegetables, using the mayonnaise solely to accent these other flavors. This colorful salad is served at nearly every tapas bar; it is even served free at times with an order of beer.
In a saucepan, cook the potatoes and carrot in lightly salted water. Bring to a boil and allow to simmer until almost tender. Fold in the peas and beans, and cook until all the vegetables are tender. Drain the vegetables and transfer them into a serving platter. Add the onion, pepper, gherkins, baby capers, anchovy-stuffed olives, and egg slices.
In a separate bowl, thoroughly combine the mayonnaise, lemon juice and mustard. Add this mixture to the serving platter, mixing well to ensure all the ingredients are coated. Sprinkle with pepper and toss. Garnish with chopped parsley and refrigerate. Allow to stand at room temperature for about 1 hour immediately before serving to enhance the salad's flavor. As any dish made with mayonnaise, ensaladilla should be refrigerated and will not keep for more than 1 to 2 days.