Place the butter in a cast iron pot (cast iron works best as it maintains a steady heat, but any pot will suffice). Once the butter is melted, add the onion, season with salt and pepper, sprinkle over the sugar and add the vinegar.

Turn the heat up high and continue to mix well until the onions soften slightly. Now turn the heat down to its lowest setting, cover the onions with a cartouche (lid of greaseproof paper) and leave to simmer ever so slightly for up to one hour. Make sure to gently stir the onions every 10 minutes.

Once the onions are caramelised, spoon them into a sieve and set aside to drain off the excess butter.

Preheat an oven to 200 degrees Celsius/gas 6. Place the slices of sourdough onto trays. Divide the onions between each slice and spread them out to cover the slices of bread. Add three slices of blue cheese to each one, on top of the onions, and place the tartine into the oven for approximately 8-10 minutes.

While the tartine is in the oven, drizzle the olive oil over the watercress, add the lemon juice, season and mix well. To serve, place the tartine on a serving board or plate with a bowl of the dressed watercress.