Get a large wide saucepan over a medium heat with the butter.
Peel and slice the onions as finely as possible. Add to the hot pan along with a large pinch of salt and pepper and the bay leaf. Stir well and cook over a high heat for 5 minutes until the onions start to release some juices. Turn the heat down to low, cover and allow to cook for 10 minutes, stirring occasionally. Once the onions have released their juices and started to colour remove the lid and continue cooking very gently for 1 hour, stirring occasionally until deep golden brown.
Tip in the flour and stir to combine. Finely chop the thyme and add that along with the brandy. Turn the heat back up to high and cook until the brandy has reduced by half. Repeat with the white wine and then add the beef stock. Simmer for 1 hour over a low heat. Remove the bay leaf once cooked. Taste and adjust the seasoning, allow the soup to cool and then refrigerate overnight.
For the Croutes
Preheat drawer 1 of the Airfryer to Bake at 160℃ and drawer 2 to Reheat at 140℃ for 6 minutes. Cut 2x 2cm thick slices from the baguette. Fill 2 ramekins with the chilled soup. Once preheated, toast the baguette slices and reheat the soup in the ramekins at the same time.
Remove the toasted bread slices from the Airfryer and lay one slice of toast on top of each soup ramekin. Generously grate over the gruyere cheese. Replace the drawer into the Airfryer.
Set Airfryer to Grill at 200℃ for 3 minutes or until the cheese is golden and bubbling.
For the Watercress salad
Finely chop the cornichons and add to a large serving bowl. Tip in the mustard, olive oil and cider vinegar. Season with salt and pepper and then mix well with a fork. Add the watercress and toss together.