Zest and halve lemon. Peel and finely chop garlic. Trim and thinly slice scallions, separating whites from greens.
Pat chicken dry with paper towels and then place between 2 pieces of plastic wrap. Pound until about ½in thick. Season all over salt and pepper, then sprinkle both sides with flour to evenly coat. Shake off excess.
Heat large drizzle of oil in a large pan over medium-high heat. Add chicken; cook until browned, about 4-5 minutes each side, adding another large drizzle of oil when flipping.
Turn off heat, transfer to plate, wipe out pan.
Heat large drizzle of oil in same pan over medium heat. Add scallion whites and garlic; cook until fragrant, about 1-2 min. Add water, stock concentrate, and lemon juice. Cook, stirring occasionally, until slightly thickened, about 1-2 min.
Remove from heat, whisk in sour cream, lemon zest, and butter. Season with pepper.
Plate chicken, top with sauce, and garnish with scallion greens.
