Peel, then mince or grate garlic. Peel, then cut onion into ¼-inch pieces. Zest, then juice half the lemon. Cut remaining lemon into wedges. Roughly chop spinach.
Heat a large non-stick pan over medium-high heat. While the pan heats, pat chicken dry with paper towels. Season with salt and half the lemon-Pepper Seasoning. When the pan is hot, add ½ tbsp (1 tbsp) oil, then chicken. Cook until golden-brown, 1-2 min per side. (NOTE: Chicken will finish cooking in step 4.) Transfer chicken to a plate. Carefully wipe the pan clean.
Heat the same pan over medium. When hot, add 1 tbsp (2 tbsp) butter, then swirl the pan until melted. Add onions. Cook, stirring occasionally, until slightly softened, 1-2 min. Add garlic and remaining Lemon-Pepper Seasoning. Cook, stirring often, until fragrant, 30 sec.
Stir in rice, broth concentrate, half the lemon juice, ¼ tsp (½ tsp) salt and 1 ¼ cups (2 ½ cups) water. Bring to a boil over high. Arrange chicken and any juices from plate over top. Reduce to medium-low. Cover and cook until rice is tender, liquid is absorbed and chicken is cooked, 15-18 min.** (TIP: Rice will abdsorb extra broth while it rests.) Transfer cooked chicken to a clean cutting board. Sprinkle spinach over rice. Remove from heat, cover, then set aside for 3-5 min.
Meanwhile, cut tomato into ¼-inch pieces. Add tomatoes, half the feta, ¼ tsp (½ tsp) lemon zest, ½ tsp (1 tsp) oil, ⅛ tsp (¼ tsp) sugar and remaining lemon juice to a small bowl. Season with pepper, to taste, then stir to combine. Squeeze a lemon wedge over top, if desired.
Thinly slice chicken Add remaining feta to rice, then stir until spinach is lightly wilted, 30 sec. Divide rice between plates, then top with chicken. Spoon tomato-feta salsa over chicken.
If you've opted to get chicken thighs, cook them in the same way the recipe instructs you to cook the chicken breasts.