My Cheeseburger Chowder

Name: My Cheeseburger Chowder
Ingredients: 450 g 20% fat beef mince; 2 tbsp any oil; 2 tbsp Bovril; 50 mls water; 1 medium onion (finely chopped); 250 mls water (roughly); 1 tbsp plain flour; pinch salt; pinch pepper; 2 ripe tomatoes (sliced); ¼ iceberg lettuce (chopped); 4 tbsp chopped gherkins; 4 slices plastic cheese; 4 sesame burger buns; Yellow mustard