In a small bowl combine 1/2 cup finely chopped parsley, 1/4 cup cilantro (if using), 2 minced garlic cloves, 1 tablespoon red wine vinegar, 3 tablespoons extra virgin olive oil, 1/2 teaspoon dried oregano, 1/4 teaspoon red pepper flakes, 1/2 teaspoon salt, 1/4 teaspoon black pepper, and 1/2 tablespoon lemon juice. Stir until well combined. Taste and adjust salt, acid, or oil as needed. Let rest at room temperature for at least 10 minutes to marry flavors.

Sandwich / Plate

Prepare steak

Pat the thinly sliced steak dry with paper towels. Season both sides with 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper. Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering. Working in one or two quick batches to avoid crowding, sear the steak slices 30–60 seconds per side until browned and cooked through (thin slices cook very fast). Transfer cooked steak to a plate and keep warm.

Assemble and melt cheese

Preheat oven broiler or use a hot skillet with a lid. If making a sandwich: halve the baguette lengthwise and lightly butter cut sides. Toast under the broiler or in a hot skillet 1–2 minutes until golden. Arrange the seared steak evenly on the bottom half of the baguette, top with the 4 ounces mozzarella evenly over the steak, and place under the broiler for 1–2 minutes until cheese is melted and bubbly. If plating: arrange steak on a plate, top with mozzarella and place briefly under the broiler (on a baking sheet) 1–2 minutes until cheese melts.

Finish sandwich / plate

If sandwich: top melted-cheese steak with 2 cups washed lettuce leaves and tomato slices. Spoon 1–2 tablespoons chimichurri over the steak or serve chimichurri on the side. Close sandwich and slice into portions. If plating: serve the steak with a spoonful (1–2 tablespoons) of chimichurri on top or on the side and arrange lettuce and tomato alongside.

Side salad (if serving plated)

In a bowl combine 3 cups mixed salad greens, 1/2 sliced cucumber and 2 tablespoons thinly sliced red onion. Whisk together 1 tablespoon olive oil, 1/2 tablespoon red wine vinegar, 1/4 teaspoon salt and 1/4 teaspoon black pepper; toss with the greens just before serving.