Raclette burgers

Name: Raclette burgers
Cook time: 30 min
Total time: 30 min
Cuisine: American
Ingredients: red onion 1 small, tough outer layers peeled and discarded; steak mince 400g; celery salt 1 tsp; flat-leaf parsley a handful, finely chopped; prosciutto 4 slices; oil for frying; raclette (or use another melty cheese, such as taleggio) 4 slices; shredded lettuce to serve; brioche-style burger buns 4, toasted; mayonnaise 4 tbsp; sweet american mustard 3 tsp; cornichons 6, chopped; pickled onions 4, chopped