Smashed burgers with jalapeño rarebit

Name: Smashed burgers with jalapeño rarebit
Cook time: 40 min
Total time: 40 min
Cuisine: American
Ingredients: 400g beef mince (around 10% fat); 1 small onion, grated; 1 tsp garlic salt; oil, for frying; mayonnaise, to serve; 4 brioche-style burger buns, toasted and ground; soft lettuce leaves; red onion, slices to serve; gherkin, slices to serve; 25g butter; 25g plain flour; 150ml Mexican lager; 1 tbsp American mustard; 100g cheddar, grated; 2 tbsp pickled jalapeño, chopped