Directions

1. Season chicken with salt and pepper. In a large skillet, heat oil over medium-high heat. Add chicken; cook until browned, turning once, 8 to 10 minutes.
2. Add 1/4 cup water. Reduce heat to medium-low; cover, and simmer until chicken is cooked through, 4 to 6 minutes. Transfer chicken to a plate; cover to keep warm.
3. Make mustard sauce: Add minced shallot to same pan; cook over medium heat, stirring occasionally, until soft, about 3 minutes. Stir in mustard and vinegar; cook 1 minute. Remove from heat. Add butter; swirl pan until incorporated. Stir any accumulated chicken juices into sauce.
4. In a medium bowl, toss spinach with half the mustard sauce. Place chicken on a serving plate; drizzle with remaining sauce. Serve spinach on the side.