Carefully slice a horizontal pocket into the thickest part of the chicken breast, being careful not to cut all the way through.
Finely mince the garlic and sauté the spinach in a small pan with a teaspoon of water until completely wilted, then squeeze out any excess moisture.
In a small mixing bowl, stir together the wilted spinach, Greek yogurt, crumbled feta cheese, and minced garlic until well combined.
Spoon the spinach mixture into the pocket of the chicken breast and secure the opening with two or three toothpicks.
Season both sides of the stuffed chicken breast evenly with the sea salt, black pepper, and onion powder.
Heat the olive oil in a medium skillet over medium heat and sear the chicken for 6 to 8 minutes per side until the internal temperature reaches 165 degrees Fahrenheit.
While the chicken is searing, steam the broccoli florets in a steamer basket for 5 minutes until they are tender-crisp and bright green.
Remove the toothpicks from the chicken and serve immediately alongside the steamed broccoli.