Dark, leafy greens are a welcome addition to this classic stuffing.
Preheat oven to 325F. Slice the bread into ¼”-thick slices. Place on a parchment-lined Sheet Pan and bake until dry to the touch, 7-10 minutes.
Increase oven temperature to 375F. In a medium Saucepan, bring 2 quarts chicken stock to a boil. Reduce the heat to a simmer and reduce the stock until 3 cups remain and set aside.
Place a Saute Pan over medium-low heat and warm olive oil. Add leeks, onions, celery, garlic, and 1 tsp. salt. Sweat until translucent, about 8 minutes. Add the sherry vinegar. Cook until liquid is reduced by half. Add the sage, 3 thyme sprigs and unreduced ¼ cup chicken stock. Cover and simmer for 10 minutes.
Uncover and reduce the liquid by half again. Incrementally add the chard and kale, folding in with the onion mixture to wilt the greens. Season with 1 tsp. salt, cover, and simmer for 10–15 minutes, adding a tablespoon or 2 of water if it starts to get too dry. Remove from heat and let cool. Taste and adjust seasoning.
Line the bottom of the 9x13 Baking Dish with a single layer of bread slices, breaking pieces of bread to fill in the cracks to cover the whole base of the dish. Pour 1 cup of reduced stock over the bread and allow it to soak in. Place half the greens and onion mix and spread out evenly. Top with another layer of bread.
Pour another 1 cup of reduced stock and allow the bread to soak. Place the remainder of the greens and onion mix and spread out evenly. For the last layer, break up the bread into smaller pieces (about 1.5”) and arrange like a mosaic. Pour the remainder of the stock and allow to soak through.
Dot the top with the butter and scatter the remaining 5 thyme sprigs. Place into the oven and bake until golden brown, about 30 minutes. Top with flaky salt, if desired.