2 For the meatballs: Soak bread in lukewarm water. Peel onion, chop finely and sweat in pan with 1 tablespoon hot oil until glazed. Remove from heat and let cool. Rinse chile, half lengthwise, remove seeds and finely chop. Rinse parsley, shake dry and finely chop leaves.
Knead ground meat with egg, mustard, onion, chile and parsley and season with salt and pepper. Form 8 small meatballs from mixture and fry each in hot pan with butter until golden brown. Lower heat and simmer meatballs until done.
3 Halve hamburger buns horizontally and bake in preheated oven until slightly toasted. Rinse lettuce leaves, shake dry and pluck small. Rinse tomatoes, remove stems and cut into thin slices. Place lettuce leaves on bottom buns, cover with 2 tomato slices, set a meatball on top and cover with top bun. Stack 2 sliders on top of each other and secure with long wooden skewers. Serve on plate with rich ketchup as desired.