Pat the chicken breasts dry with paper towels and place them in a large mixing bowl. Add the olive oil, minced garlic, fresh lemon juice, chopped parsley, oregano, thyme, salt, and black pepper. Toss until every piece is evenly coated, then let the chicken marinate for at least 20 minutes or up to 4 hours in the refrigerator for deeper flavor.
Heat a large skillet over medium-high heat. Place the marinated chicken into the hot skillet and cook for 5–7 minutes without moving it so a golden crust develops. Flip and cook for another 5–7 minutes, or until the thickest part reaches an internal temperature of 165°F (74°C). Transfer the chicken to a cutting board and allow it to rest for at least 5 minutes before slicing it against the grain.
While the chicken is resting, cook the brown rice according to the package directions if it has not already been prepared. Fluff the cooked rice with a fork and allow it to cool slightly.
Prepare the garlic herb yogurt sauce by whisking together the Greek yogurt, chopped fresh dill, Dijon mustard, a squeeze of fresh lemon juice, a pinch of salt, and black pepper until smooth and creamy. Refrigerate the sauce until ready to serve.
Wash and thoroughly dry the cucumber, cherry tomatoes, spinach, and any additional herbs. Dice the cucumber, halve the tomatoes, and slice the avocado just before assembling the bowls to keep it fresh.
Divide the cooked brown rice evenly among four serving bowls or meal prep containers. Arrange the sliced garlic herb chicken over the rice, then add the spinach, cucumber, cherry tomatoes, and avocado in separate sections. Sprinkle with crumbled feta cheese if using.
Drizzle the garlic herb yogurt sauce over the bowls just before serving, or store it separately if meal prepping. Garnish with extra chopped parsley, dill, and a squeeze of fresh lemon juice for the freshest flavor.
Enjoy immediately while the chicken is warm, or refrigerate the assembled bowls for up to four days. Reheat only the chicken and rice before adding the fresh vegetables and yogurt sauce if serving later.