Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Combine the dry quinoa with 0.66 cups of water in a small pot, bring to a boil, then cover and simmer for 15 minutes until fluffy.
Place the chicken breast on the baking sheet, drizzle with olive oil, and season evenly with dried oregano, sea salt, and black pepper.
Roast the chicken in the oven for 20 to 25 minutes or until the internal temperature reaches 165°F.
While the chicken cooks, grate half of the cucumber and squeeze out excess moisture, then stir it into the Greek yogurt with lemon juice, minced garlic, and fresh dill.
Slice the remaining cucumber, halve the cherry tomatoes, and finely dice the red onion to prepare the fresh vegetable topping.
Slice the roasted chicken and assemble the bowls by layering quinoa, chicken, fresh vegetables, and a generous dollop of tzatziki, finishing with crumbled feta.