John Torode's big burger

Name: John Torode's big burger
Prep time: 25 min
Cook time: 30 min
Total time: 55 min
Cuisine: American
Ingredients: 1 tbsp olive oil; 2 tsp ground cumin; 1 bay leaf; 2 garlic cloves crushed; 2 small shallots chopped; 1 red chilli chopped; 1 celery stick, chopped; 500g very ripe tomatoes roughly chopped; 2 egg yolks; 50g oyster sauce; 6 slices bread made into crumbs, then soaked in a little water; 1kg good-quality beef mince, not lean; 8 slices gruyères (optional); 4 burger buns warmed; little butter; 4 tbsp good-quality mayonnaise; squeezy mustard; 1 beef tomato sliced; few cooked rashers streaky bacon; Little Gem lettuce leaves; guacamole see 'goes well with'; few gherkins