Burmese creamed corn with onions

2 onions, 1 sliced and 1 diced

4 tbsp of groundnut oil, or another neutral-tasting oil

500g of sweetcorn kernels, tinned in unsalted and unsweetened water

1 tbsp of condensed milk, or double cream

1 tbsp of tapioca starch, heaped, mixed with 3 tbsp water to form a paste