Tuna Melt with Ketchup from The Tinned Fish Cookbook: Easy-To-Make Meals from Ocean to Plate by Bart

Ingredients

• 1 small red onion, diced
• Salt and pepper, to taste
• One 6-ounce (160 g) tin of tuna in sunflower oil, drained
• 2 tablespoons olive oil
• 1½ tablespoon tomato purée
• One 14-ounce (400 g) can peeled plum tomatoes
• 3 tablespoons mayonnaise
• 4 slices of bread, preferably rustic
• 1 scallion, white and light green parts, finely chopped
• 1 tablespoon red wine vinegar
• 1 tablespoon dark brown sugar
• ¼ bunch of flat-leaf parsley, leaves only, coarsely chopped
• 4 slices of cheddar cheese
• 1½ tablespoon (20 g) butter
• Tabasco sauce
• 1 garlic clove, minced