1. Melt the butter in a small skillet over medium heat and cook the onions, stirring occasionally, until soft, about 5 minutes. Add 1/4 teaspoon salt, the curry powder and two-thirds of the garlic and continue cooking, stirring, until fragrant, about 30 seconds. Transfer the mixture to a large bowl and let cool.
2. Lightly oil a rimmed baking sheet. Add the beef, 1/4 cup of the yogurt, the breadcrumbs, tamarind chutney, paprika, cayenne , if using, 1 1/4 teaspoons salt and the eggs to the onion mixture. Combine the ingredients using your hands. Form into 1 1/4- inch balls, rolling them lightly between your hands to shape. Place on the oiled baking sheet, cover loosely with plastic wrap and refrigerate at least 30 minutes or overnight.
3. Preheat the oven to 425 degrees F. Bake the meatballs until browned, about 20 minutes.
4. Meanwhile, stir together the remaining 3/4 cup yogurt, remaining garlic, 1/2 teaspoon salt and cilantro in a small bowl. Arrange each meatball on top of a piece of lettuce on a platter. Top with grated cucumber and a bit of tamarind chutney and serve with the yogurt dipping sauce.