Grilled Tuna Cheddar & Onion Sandwiches

• 12 nonstick frying (pan)
• 2 sandwiches
• 1 onion
• 1 garlic clove
• 1/2 broth
• 1/2 remaining broth
• 110 gr cheddar cheese
• 120 ml chicken broth
• 50 gr celery
• 30 ml mayonnaise