Reduce onion shallot, wine and vinegar to 2–3 tbsp of liquid. On low heat, whisk in 1 tbsp cold butter at a time. Add salt, strain. Use immediately.

Make reduction - In a small saucepan over medium heat, add the onion shallot, white wine and vinegar. Let it simmer gently until it reduces by 2/3 (about 5 minutes once it starts simmering). You should have just 2 to 3 tablespoons of the liquid left no including the onion shallot. If using cream (see Note 4), add it now and continue simmering until the mixture reduces again to just 2 to 3 tablespoons of liquid. (Note 5)

Add COLD butter - Turn the heat to low. That is very important because if it’s too hot, your sauce will split (you don't want the mixture to even reach a simmer, use your smallest burner). Add the cold butter, one heaped tablespoon at a time while whisking briskly and constantly. (Note 6)

Emulsify sauce - Let each addition melt before adding more. The sauce will thicken as you add the butter and will become of a pale milky yellow colour. This should take around 5 minutes. Add the salt in at the end. (Note 7)

Sauce consistency - Keep whisking until all the butter is incorporated, the sauce should be slightly thickened like pouring cream with a pale yellow colour. It should lightly coat the back of a spoon.

Strain - Strain the sauce through a fine sieve into another small saucepan, pressing on the eschalots to extract all the flavour. (Note 9)

Warm and serve - Place the saucepan over LOW heat on your smallest burner to keep the sauce gently warm while you cook your protein. The heat must stay very low or the sauce will split. You only want to keep it warm, not hot. (Note 8) When needed, spoon generously over fish like I do here or chicken, or even simple steamed vegetables. Sprinkle some finely chopped chive over the top and enjoy!