Ham & cheese croquetas

Name: Ham & cheese croquetas
Prep time: 20 min
Cook time: 1 h 25 min
Total time: 1 h 45 min
Cuisine: Spanish
Ingredients: 25g butter; ½ small onion, finely chopped; 50g plain flour; 250ml milk; 140g sliced smoked ham, diced; 50g mature cheddar, coarsely grated; 50g gruyère, finely grated; 1 tsp Dijon mustard; 2 tbsp double cream; 2 large eggs; 140g fine dried breadcrumbs; sunflower oil, for deep frying; 300g ripe tomatoes, roughly chopped; 1 long red chilli, finely chopped (deseeded if you don't like it too hot); 1 small red onion, finely chopped; 4 large garlic cloves, crushed; 100g demerara sugar; 100ml red wine vinegar