Season both sides of the fillet. Note, salt at the very last minute prior to cooking to prevent the salt from “burning” the fish.
Coat the bottom of a non-stick pan with extra virgin olive oil and turn to medium heat. Add butter. When the oil and butter start to sizzle, add cod fillets and sear two to three minutes. Turn fish over. Tilt the pan and collect butter and oil and, with a spoon, baste your fish. Continue to cook for another two to three minutes. Pro tip – turn when the side that is in contact with the oil turns opaque. You will know that the cod is cooked when it flakes easily when poked with a fork.
Over a double boil, beat egg yolks and warm water (rule of thumb, 1 tablespoon of water for every egg yolk) until thick and creamy. Look for very soft peaks.
Melt the butter. You may use the microwave.
Once the yolks reached the desired thickness, remove from heat. You may place a warm towel under your bowl. Whenever making any kind of emulsion, the addition of fats needs to be gradual – start with a few drops to get the emulsion started.
Continue streaming in butter until it is fully incorporated.
Season with salt and pepper. Add herbs, if desired.
Add tomato paste. Mix to combine.
Finish with lemon juice. Adjust final consistency with a little bit of warm water to lighten the sauce and fluidity. Keep warm over a double boiler until ready to serve.