Start the soup base. Heat the olive oil in a large, heavy-bottomed saucepan over a medium flame until it shimmers and smells grassy. Add the diced onion and cook for 6-7 minutes, stirring occasionally, until it turns translucent and smells sweet—you want it soft, not coloured.

Build the flavour. Stir in the minced garlic and tomato paste. Cook for 2 minutes, scraping the bottom of the pan. You’ll know it’s ready when the paste darkens to a deep brick-red and releases a rich, concentrated aroma that fills your kitchen.

Simmer the soup. Pour in the tinned tomatoes, stock, oregano, and caster sugar. Use a wooden spoon to break up the tomatoes. Bring it to a gentle boil—you’ll see lazy bubbles breaking the surface—then reduce the heat to low. Let it simmer uncovered for 15 minutes, until it thickens slightly and the colour deepens to a rich sunset orange.

Blend and finish. Remove the pan from the heat. Using an immersion blender, purée the soup until it’s completely smooth and silky—no lumps remain. Stir in the balsamic vinegar and double cream (if using). Season with salt and pepper. Keep the soup warm over the lowest heat while you make the sandwiches.

Prepare the bread. In a small bowl, mix the softened butter with the Dijon mustard until well combined. Spread this butter-mustard mixture evenly on one side of each bread slice. This isn’t just for flavour—it ensures the bread toasts evenly and develops that deep amber crust.

Assemble the sandwiches. On the unbuttered side of four slices, pile the grated cheddar and Gruyère. Don’t be shy—you want a generous, even layer. Top with the remaining bread slices, buttered side facing up. Press down gently to seal.

Cook to golden perfection. Heat a large, non-stick frying pan or cast-iron skillet over a medium-low heat. Place the sandwiches in the pan (work in batches if needed—do not overcrowd). Cook for 3-4 minutes per side. Listen for a steady, gentle sizzle—if it’s popping aggressively, the heat is too high. The bread should turn a deep, even golden brown, and you’ll see the cheese starting to ooze from the edges. When you lift a corner with a spatula, the crust should feel rigid and sound crisp.

Rest and slice. Transfer the sandwiches to a cutting board and let them rest for 1-2 minutes. This step is crucial—it allows the cheese to set slightly so it doesn’t all gush out when you cut. Slice each sandwich diagonally into two triangles.