Grilled Steak with Rosemary Lemon Butter

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

For the Lemon Rosemary Butter:

• 2 tablespoons butter at room temperature, unsalted
• 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 rosemary chopped fresh leaves