Easy Steak Sandwich Recipe with Garlic Herb Butter (30 Minutes)

Ingredients

• 6 tablespoons (85 g) unsalted butter, softened
• 2 cloves garlic, finely grated or minced
• 2 tablespoons finely chopped fresh parsley
• 1 tablespoon finely chopped fresh chives
• 1/2 teaspoon lemon zest
• 1/4 teaspoon kosher salt
• 1/4 teaspoon black pepper
• 1 lb (450 g) ribeye, strip steak, sirloin, flank, or skirt steak
• 1 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 1 tablespoon olive oil
• 1 large yellow onion, thinly sliced
• 1 baguette or 4 crusty sandwich rolls, split
• 4 slices Havarti or provolone cheese
• 1 cup (25 g) baby arugula