Ingredients

• 2 large onionsyellow or brown; very sweet reds caramelise less reliably
• 25 g butter plus extra for bread
• 1 tsp olive oil
• 1 tsp brown sugar
• 1 tsp Worcestershire sauce or balsamic vinegar
• 4 thick sourdough slicesabout 1.5 cm
• 100 g Gruyeregrated
• 80 g mature cheddargrated
• 1 tsp Dijon mustard
• 4 thyme sprigs
• 1/4 tsp freshly ground black pepper

Method

1. 01

   Caramelise the onions slowly

   Melt the 25 g butter with the olive oil in a frying pan over medium-low heat, add the sliced onions, the thyme sprigs and a good pinch of salt, and cook for 25-30 minutes. Stir every few minutes and scrape the base. They will go from white to soft to gold to deep brown and jammy. There is no version of this that takes ten minutes.

   1 tsp olive oil · 2 large onions

   The onions are the recipeTwenty-five to thirty minutes on medium-low. Brown at ten means scorched, and it tastes bitter.

2. 02

   Sharpen them at the end

   Add the brown sugar and cook for 2 minutes, then take the pan off the heat and stir in the 1 tsp Worcestershire or balsamic and the 1 tsp Dijon. Off the heat, so the acidity stays sharp rather than cooking away. Discard the thyme stalks.

   1 tsp brown sugar

3. 03

   Build the sandwiches

   Mix the Gruyere and cheddar. Butter the outside of each of the 4 sourdough slices. Build each sandwich with cheese, then onions, then more cheese. Cheese against both slices of bread glues everything together as it melts.

   100 g Gruyere · 80 g mature cheddar · 4 thick sourdough slices

   Cheese against the breadCheese on both faces, onions in the middle. Onions against bread make it damp.

4. 04

   Cook covered, on medium-low

   Cook in a dry frying pan over medium-low heat with a lid for 3-4 minutes a side, pressing gently with a spatula, until the outside is deep golden and the cheese has completely melted through. The lid is what gets the middle molten before the crust goes too dark.

5. 05

   Rest, then cut

   Rest for a minute before cutting. Straight from the pan the cheese runs straight out. Grind over black pepper and cut on the diagonal.