Heat the oil in a large skillet, add the breasts, and cook over medium heat until they are lightly colored, about 3 minutes per side. Remove from the skillet and set aside.
Add the shallots to the skillet and cook over low heat until they are translucent, about 5 minutes. Add the vinegar, raise the heat, and cook, stirring occasionally, until it has reduced to a syrupy spoonful. Whisk in the stock, cream, and creme de cassis, and simmer for 1 minute.
Return the breasts to the skillet and simmer them gently in the sauce until they are just done, about 5 minutes; do not overcook. Remove the breasts with a slotted spoon and arrange them on a heated serving platter.
Simmer the sauce gently until it has reduced and thickened slightly, 3 minutes. Add the blackberries; cook 1 minute.
Pour the sauce over the breasts and serve immediately, garnished with chervil leaves.
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Razzle Dazzle Recipes