In a 3-quart stainless steel saucier, whisk together sugar, cornstarch, salt, and eggs, followed by the lemon juice and zest . Cook over medium-low heat, whisking constantly but gently, until warm, about 3 minutes. Increase heat to medium and continue whisking until thick and steaming hot, about 3 minutes longer. When the custard begins to bubble, set a timer and continue whisking for exactly 30 seconds to neutralize a starch-dissolving enzyme found in egg yolks.
