Heat the oven to 400°F and line a rimmed baking sheet with parchment paper.
Grate the cucumber on the large holes of a box grater, wrap it in a clean kitchen towel and squeeze out the excess liquid. Stir together 1 cup of the yogurt, the cucumber, 1 minced garlic clove, 2 tsp of the dill, 1 tbsp of the lemon juice and a pinch of salt. Refrigerate until serving.
In a large bowl, combine the ground chicken, panko, egg, red onion, remaining garlic, remaining 1 tsp dill, lemon zest, 3/4 tsp salt and the pepper. Mix gently with your hands just until combined; do not overmix.
Shape the mixture into 24 meatballs, about 1 tbsp each, and arrange them on the prepared baking sheet. Brush with the olive oil.
Bake 18-20 minutes, until lightly browned and an instant-read thermometer inserted into the center of a meatball reads 165°F.
Rest 5 minutes, then serve warm with the chilled tzatziki and lemon wedges.
