Scrub the citron well. Quarter lengthwise, remove the pulp for another use, and slice the thick peel (including the white pith) into 0.5-inch-wide strips.
Place the peel in a large pot and cover with cold water by about 2 inches. Bring to a boil over medium-high heat, then reduce to a simmer for 5 minutes. Drain. Repeat this blanching twice more with fresh water each time until the peel is flexible and the bitterness is mellowed.
Combine the water and granulated sugar in a clean, wide pot. Bring to a simmer over medium heat, stirring until the sugar dissolves, 2–3 minutes, to make a 1:1 syrup. Add the blanched peel and lay a parchment or foil circle directly on the surface to keep the pieces submerged.
Simmer very gently (barely bubbling) for 15–20 minutes until the peel looks glossy and slightly more translucent; avoid vigorous boiling to prevent toughening. Remove from heat and let the peel cool completely in the syrup. Cover and rest 12–24 hours at cool room temperature.
Over the next 2–3 days, repeat the cycle: bring just to a bare simmer for 10–15 minutes, then cool and rest 12–24 hours between reheats, until the peel is fully translucent through the center and tastes sweet all the way to the core.