Start by caramelizing the onions: place the onions in a skillet over medium heat with 1 tablespoon of the oil. Saute, stirring occasionally, until the onions turn a rich, deep-golden brown, about 30 minutes.
Next, in a 1-quart or larger saucepan on medium heat, saute the shallots in the remaining 1 tablespoon oil until translucent and the edges begin to brown.
Mince the caramelized onions and add to the shallots, along with the wine and vinegar. Reduce the liquid until syrupy and almost gone. Add the cream and chipotle, and reduce the liquid, until thickened by half. Add the lemon juice and salt and pepper to taste.
Turn down the heat to the lowest setting and stir in the cold butter a little at a time until melted. Repeat until all of the butter has been incorporated. Turn off the heat (you do not want this mixture to come to a boil once the butter has been added) and re-check for salt and pepper.
Keep the sauce in a warm place until ready to serve or store in the refrigerator for up to 2 weeks.
To reheat, warm a couple tablespoons of cream in a small saucepan over low heat, and stir in the cold sauce a little bit at a time until completely incorporated (do not boil).
Serve with fresh oysters.
The heat of chile can vary; feel free to add a bit more chipotle if desired.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.