Beurre noisette
"At its simplest, beurre noisette is a one-ingredient recipe, requiring just butter. Typically, you'd cut the butter into small pieces and melt it slowly in a pan. Unsalted butter works better, as it foams less, so it's easier to monitor the cooking process. You cook it slowly on a stove for a few minutes until it turns light brown; at this point, it's caramelizing and develops a rich, nutty flavour. Be sure to take it off the heat so it doesn't overcook and burn, which will make it bitter."
"Since lemon and fish are an excellent pairing, you can add a spoonful of lemon juice at the end to transform the brown butter into a delectable sauce. You can season it with salt and pepper if you like; just be sure to prepare it close to when you want to serve your food, as it will firm up if you leave it standing. Before you drizzle the sauce over your fish, add any other herbs or seasonings you want to experiment with. Shallots, capers, and parsley are also excellent additions that will go with fish."
"Beurre noisette is a key part of French culinary culture. It may not be one of the so-called French "mother sauces" (those are all a bit more complicated in terms of ingredients), but it's common in a lot of French concoctions. The name literally means "hazelnut butter," but as you've likely realized, hazelnuts are nowhere to be found in this sauce. Rather, this refers to the hazelnut color of the finished product, and perhaps the nutty flavor browning it imparts. That colour and flavour is a result of the Maillard reaction, the chemical reaction between sugar and amino acids when you cook food. It creates deeper, richer flavours — for example, the caramelized crust you might get when searing steak."
Recipe and pictures - www.chowhound.com/1920186/fancy-two-ingredient-seafood-sauce/