Made from unripe wine grapes, verjus (see Note) is high in acid but fresh-tasting. It’s a killer poaching liquid for lean meats.
Combine 3 cups water, chicken, peppercorns, 2 cups verjus blanc, herb sprigs, and 1 tablespoon salt in a large pot. Bring to a simmer over medium-high; reduce to low, and cook, uncovered, until an instant-read thermometer inserted in thickest portion of chicken registers 155°F, 25 to 30 minutes. Using tongs, transfer chicken to a plate; let cool 30 minutes. While chicken cools, pour about 1 1/2 cups poaching liquid through a fine wire-mesh strainer into a medium bowl; discard solids. Set 3/4 cup strained poaching liquid aside. Discard remaining poaching liquid.
Remove skin from chicken; carefully remove breast meat, whole, from bones. Discard skin and bones. Cut chicken breasts crosswise into 1/2-inch-thick slices; place in a 13- x 9-inch baking dish.
Whisk together olive oil, chopped herbs, remaining 1 1/2 teaspoons salt, and reserved 3/4 cup strained poaching liquid. Pour herb marinade over chicken in baking dish; cover with plastic wrap, and refrigerate 8 hours or up to overnight.
Let chicken come to room temperature, about 1 hour. Transfer to a serving platter, and spoon herb marinade over top. Drizzle with remaining 1 1/2 tablespoons verjus blanc, and sprinkle with flaky sea salt.
If you can’t find verjus, substitute 4 cups water and 1 cup of crisp white wine for the poaching liquid. For serving, drizzle with 1 tablespoon white wine vinegar.
Lemony, lightly grassy Sancerre.