Raclette burgers

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