Flank steak rolls with cheese and prosciutto - Le Coup de Grâce

Ingredients

• 1/2 onion yellow onion, finely chopped
• 3 large garlic cloves, finely chopped
• Olive oil
• A 1 kg piece of flank steak, butterfly cut open
• 100 g sliced ​​prosciutto
• 1 cup arugula
• 250g Le Cantonnier raclette cheese
• Salt and pepper from the mill