A perfectly roasted chicken is what many cooks dream of as quite possibly their last meal, and many more before then. It's Sunday dinner pared down to its bare essentials; just juicy, tender meat and crisp, crackling skin carved away by fork and knife.
Put the black peppercorns, Sichuan peppercorns, if using, and coriander seeds into a saucepan and toast over medium-low heat, swirling the pan, until fragrant, 2-3 minutes. Add 2 cups of the water and the salt, increase the heat to medium-high, and stir until the salt is completely dissolved. Remove from the heat and add the remaining water and the chile. Let cool to room temperature.
Rinse the chicken well under cold water. Transfer to a tall narrow plastic container just large enough to hold it. Pour the brine over the chicken and adjust the bird if necessary so it is completely immersed. Refrigerate for at least 2-3 hours, or preferably overnight.
Meanwhile, in a medium-sized saucepan, whisk the egg yolks, sugar, and lemon zest and juice until combined. Put the pan over low heat and stir constantly with a wooden spoon until the mixture is very thick and hot, but just below a simmer, about 15 minutes.
Remove from the heat and whisk in the butter a little at a time until smooth. Using a rubber spatula, press the mixture through a fine-meshed strainer set over a small bowl to remove the zest. Cover and let stand until cool or refrigerate if not using right away.
Position a rack in a roasting pan. Remove the chicken from the brine and rinse well under cold running water. Pat the chicken completely dry with paper towels and set it on the rack in the pan. Let the chicken come to room temperature, 20-30 minutes.
Preheat the oven to 450 degrees.
Coarsely crush the black peppercorns and coriander seeds with a mortar and pestle or on a cutting board with the bottom of a small pan. Transfer to a small bowl and stir in the rosemary and sage. Add three-quarters of the spice mixture to the lemon curd, along with 1 tablespoon of the water, and stir to combine.
Pat the chicken again with paper towels to make sure it is completely dry. Gently loosen the skin by sliding your fingers between the breast and skin and working them down to the thighs and drumsticks. Spoon half the lemon curd under the breast skin and use your hands to spread the curd evenly under the loosened skin all the way to the legs.
With kitchen twine, tie the legs together tightly. Brush the remaining curd evenly over the surface of the chicken, coating the whole bird. Sprinkle the remaining spice mix evenly over the chicken. Return to the rack in the pan and pour the remaining water into the pan.
Roast the chicken for 30 minutes. Reduce the temperature to 325 degrees and continue roasting until the chicken is golden brown and the juices run clear when the thigh is pierced, 1 ½-1 ¾ hours. Add water ½ cup at a time to the pan if it starts to dry out. Let the chicken stand for 10-15 minutes before carving.