Preheat an outdoor grill for high heat, and lightly oil grate.
Cut a slit in steak to create a pocket that is open on one side only. In a cup, mix together 1 tablespoon olive oil and lemon juice; brush over both sides of fish. Set aside.
In a small skillet, heat 1 teaspoon olive oil and garlic over medium heat. Cook spinach in oil until wilted. Remove from heat, and stuff into pocket. Place feta in pocket over spinach.
Arrange fish on grill, and cook for 8 minutes. Turn over, and continue cooking until cooked through.