Marinate the chicken by combining olive oil, garlic powder, onion powder, dried oregano, paprika, salt, and pepper in a bowl. Add chicken thighs and coat well. Cover and marinate in the refrigerator for at least 30 minutes.
Preheat grill or skillet over medium-high heat. Cook marinated chicken thighs for 6-7 minutes on each side until fully cooked (internal temperature of 165°F). Let rest for 5 minutes before slicing.
Prepare tzatziki sauce by combining Greek yogurt, grated cucumber, minced garlic, lemon juice, and a pinch of salt in a bowl. Mix well and adjust seasoning to taste.
Assemble the bowl by dividing cooked brown rice among four bowls. Top with sliced chicken, fresh vegetable mixture (cherry tomatoes, cucumber, red onion), and a generous dollop of tzatziki sauce. Garnish with chopped parsley.