Place onion, garlic and oregano in a food processor and pulse for a few seconds until the onions are finely chopped. In a mixing bowl, combine onion mixture with ground meat, add salt and pepper and mix thoroughly. Shape into 20 meatballs. In a large skillet over medium heat, warm the olive oil and fry the meatballs, turning frequently, until cooked through (about 10-12 minutes). If you can’t fit all the meatballs into the skillet at once, fry in two batches. Transfer to a serving dish and let cool.
In a small, electric blender, combine kefir, feta cheese, olive oil, mint leaves and garlic and process to combine. If the kefir is very tart, add a little honey to balance the acidity. If desired, you can add a little water or milk to thin the dressing; it should have the consistency of whipping cream. Season to taste with salt and pepper.
Spread out the shredded lettuce on a large serving platter (or individual serving plates) and top with sliced cucumber, onion, and halved tomatoes. Set the meatballs on top and spoon over the kefir dressing. Garnish with fresh dill fronds and serve immediately.