What ingredient gives the warm vinaigrette in Salade Lyonnaise its distinctive savoriness?

Answer

Bacon fat

The crisp *frisée* lettuce in the Salade Lyonnaise is tossed in a warm vinaigrette which crucially incorporates bacon fat, lending a distinctive savoriness to the salad.

What ingredient gives the warm vinaigrette in Salade Lyonnaise its distinctive savoriness?
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