Preheat the oven to 350°F (175°C).
Spread the fresh spinach in an even layer across the bottom of a large casserole or baking dish.
Place the chicken breasts in a single layer on top of the spinach and sprinkle both sides evenly with the Greek seasoning.
Arrange the halved cherry tomatoes, thinly sliced red onion, kalamata olives, sliced pepperoncini, pressed garlic, and crumbled feta evenly over the chicken.
Squeeze the juice of 1 lemon evenly over the assembled ingredients.
Bake in the preheated oven for 35–45 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and is cooked through.
Remove from the oven and let rest a few minutes, then serve warm.