Cut beef fillet into 3-4 cm pieces. In a large bowl, combine meat with salt and pepper to taste, add onion strips and dry red wine, mix well. Cover with cling film and refrigerate for 3 hours.
Place plums in a saucepan with water, bring to a boil, then simmer over medium-low heat for 20 minutes. Remove plums from pan and set aside, keeping the cooking liquid.
Cut cooled plums in half and remove pits. Place plum flesh in a food processor with 2 garlic cloves, chopped cilantro, and roughly 5 tablespoons of the plum cooking liquid. Process until smooth. While processing, gradually add more cooking liquid until the sauce reaches a creamy consistency. Season with salt to taste and a pinch of cayenne pepper.
Preheat grill to high heat.
Melt 15g butter on the stove over medium heat.
Thread beef pieces onto skewers, brush with melted butter, and place on the preheated grill. Cook for 10-15 minutes, turning occasionally, until browned.
Serve kebabs with salad leaves, plum sauce, and lemon wedges.