Grilled Steak with Rosemary Lemon Butter

• 2 lbs beef tenderloin steaks 4 pieces, (otherwise called filet mignon)
• 1 teaspoon kosher salt (or to taste)
• 1 teaspoon coarse ground black pepper
• 1/2 teaspoon onion powder
• 1/2 teaspoon dried whole rosemary leaves
• 1 teaspoon olive oil (if using a stove top grill pan)

For the Lemon Rosemary Butter:

• 2 tablespoons unsalted butter at room temperature
• 1/4 teaspoon lemon zest
• 1/8 tsp kosher salt (or to taste)
• 1/8 tsp black pepper (or to taste)
• 1 pinch red pepper flakes (optional but recommended)
• 1 teaspoon chopped fresh rosemary leaves