This is quoted from Heston Blumenthal's book
"When I was researching the perfect risotto. I visited the kitchen of Gualtiero Marchesi, the first Italian chef to gain three Michelin stars and regarded by many as the father of modern Italian cooking. In his risotto Milanese, Marchesi didn't start by softening some finely chopped onion because, he said, he wanted to keep the flavour of the rice and saffron " as pure as possible ". And so instead he added acidulated butter right at the end of cooking. "
100g peeled and sliced onions (approx. 1-2 onions)
185g Dry white wine
300g White wine vinegar
250g Unsalted butter, cubed
1. Place the onion, wine and vinegar in a saucepan over a high heat and reduce the liquid by three-quarters.
2. Remove from the heat and whisk in the butter a little at a time until well emulsified
3. Leave infuse for 20 minutes at room temperature, then pass through a fine sieve, using the whisk to help push through the solids through
4. Pour into a tub and keep refrigerated until required (for up to a week).