Grate the cucumber and squeeze out excess moisture using a clean towel or paper towel.
In a bowl, combine the Greek yogurt with the cucumber, minced garlic, dill, olive oil, salt, and pepper.
Mix until smooth and creamy.
Refrigerate while preparing the rest of the dish.
Season the chicken with salt and pepper, drizzle with lemon juice and 1 tablespoon olive oil. Add lemon zest and chopped dill. Let marinate for 20–30 minutes.
Cook the rice or quinoa according to package directions. Stir in 1 tablespoon olive oil and a pinch of salt once cooked.
Grill or bake the chicken until fully cooked through (about 7–8 minutes per side). Let rest, then slice.
Dice the cucumber and tomato.
Assemble the bowls: layer rice or quinoa at the bottom, add chicken, vegetables, and a generous dollop of tzatziki. Garnish with fresh dill and serve.