Rinse the quinoa thoroughly in a fine mesh sieve. In a medium saucepan, combine the quinoa and chicken broth. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until liquid is absorbed. Fluff with a fork and let cool.
Pound the chicken breasts to an even thickness. Coat with olive oil and the Greek seasoning. Grill (or pan-sear) over medium-high heat for 6-7 minutes per side until the internal temperature reaches 165°F. Let rest for 5 minutes before slicing.
Grate the cucumber for the sauce and squeeze out as much water as possible using a clean kitchen towel. In a small bowl, mix the grated cucumber, Greek yogurt, minced garlic, lemon juice, dill, and a pinch of salt.
While the chicken rests, chop your cherry tomatoes, cucumber, and red onion. Halve the kalamata olives.
Divide the fluffy quinoa among 4 bowls. Top with the sliced grilled chicken, tomatoes, cucumber, olives, red onion, and a generous sprinkle of crumbled feta cheese.
Drizzle the homemade tzatziki sauce heavily over the top of the bowls. Serve with extra lemon wedges on the side.