Heat oil in a large ovenproof sauté pan over medium-high heat. Season chicken with salt, pepper and herbes de Provence. Dust with flour; tap off excess. Cook, meaty side down, 5 minutes. Turn over; cook 1 minute.
Add garlic and shallot; stir 30 seconds.
Add wine; cook 1 minute.
Add stock; cook 30 seconds.
Add lemon juice.
Transfer pan to oven.
Bake until chicken is cooked through, 9 to 13 minutes.
Transfer chicken to cutting board.
Place pan over medium heat; simmer sauce 1 minute.
Add butter; swirl pan until butter incorporates and sauce thickens slightly.
Cut each breast into 4 slices; transfer each to a plate; top with sauce and garlic.
Serve with spinach; garnish with zest, if desired.