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In a sauce pan, dump the shallot along with the Champagne, turn heat to medium and bring to a boil.
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Gently boil Champagne mixture until reduced by at least 2/3, kill the heat, strain out the shallots and return liquid to the sauce pan to cool.
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Once liquid has cooled to room temperature or there about, add the egg yolk turn burner on to the lowest possible setting and start whisking the egg yolk.
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Whisk yolk for about a minute until light and fluffy, then start gradually whisking in the melted butter. Go a few drops at the time for starters, then gradually start pouring at a slow, steady stream all while whisking constantly. Basically as if you were making a mayonnaise or Bearnaise.
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When all the butter has been incorporated, stir in a bit of salt, freshly ground pepper and lemon juice followed by about 100 milliliters of the asparagus infused cream.
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Allow sauce to warm through slowly, but kill the heat before the eggs even think about curdling, taste again for seasoning - it may need more salt, pepper and lemon.
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Keep sauce pan on the warm burner to keep sauce warm while you steam the asparagus.