Pat the chicken breasts dry with paper towels. Season both sides evenly with kosher salt and black pepper.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Add the chicken breasts to the skillet and cook for 6-7 minutes on one side without moving them, until golden brown.
Flip the chicken breasts and cook for another 6-7 minutes, or until the internal temperature reaches 165°F (74°C). Remove the chicken from the skillet and set aside to rest.
Reduce the heat to medium and add the butter to the same skillet.
Once the butter has melted, add the minced garlic and red pepper flakes. Sauté for 30 seconds until fragrant, stirring constantly to prevent burning.
Add the fresh spinach leaves to the skillet and cook, stirring frequently, until the spinach is wilted, about 2-3 minutes.
Remove the skillet from heat and stir in the fresh lemon juice.
Serve the pan-seared chicken breasts on plates alongside the garlic spinach.