Tonno Sott'olio (Homemade Marinated Tuna in Olive Oil)

Ingredients

• 2 pounds (1-1/4-inch thick) tuna steaks
• 4 cups best-quality extra-virgin olive oil
• 2 tsps salt
• black pepper to taste
• granulated garlic to taste
• 1 cup white vinegar
• 2 red onions, peeled and thinly sliced
• 16 cloves garlic, peeled
• 8 fresh bay leaves
• 8 small red cayenne peppers
• 1 tbsp red pepper flakes, divided