Ingredients :

• The shallot — It does a job that an onion wouldn’t do quite as well. An onion would be too strong in this sauce — it would take over. The shallot melts discreetly and provides depth without imposing. Mince it very finely, almost into a brunoise, so it completely disappears during cooking.
• The lemon — Fresh, absolutely required. Bottled juice has a flat, slightly bitter taste that doesn’t do this sauce justice. What we want here is also the zest — that’s where the essential oils and the bright, floral lemon scent live. Use an untreated lemon if you can, especially for the zest.
• The heavy cream — Full-fat. This is important, really. A light cream (15%) will split upon contact with the lemon’s acidity — you’ll end up with a grainy, disappointing sauce. Cream with at least 30% fat handles the heat and acidity without flinching. Thick or liquid heavy cream, both work well here.
• Dijon mustard (optional) — Just a small spoonful at the end of cooking. It gives a slight spicy note that balances out the fat in the cream and prevents the sauce from feeling too ‘flat’ in the mouth. Don’t use it if you are serving this sauce over a very delicate fish — the mustard would be too prominent.