Beetroot and horseradish beef burger

Name: Beetroot and horseradish beef burger
Cook time: 30 min
Total time: 30 min
Cuisine: European, British
Ingredients: 400g beef mince; ½ small red onion grated; 1 tsp garlic salt; 4 slices mature cheddar; 4 burger buns halved and toasted; 4 tbsp mixed with 2 tbsp mayonnaise; soft lettuce leaves; horseradish cream; 100g cooked beetroot diced; chopped to make 2 tbsp gherkins; finely chopped to make 1 tbsp red onion