Combine sugar, water, and lemon juice in a large, heavy-bottomed saucepan over medium-high heat, swirling the pan occasionally until the sugar dissolves. Continue cooking, swirling the pan occasionally (do not stir – stirring encourages crystallization), until the sugar syrup reaches a dark amber color. Slowly (and very carefully) add the heavy cream – it will splash up and bubble ferociously. Be careful to avoid splatters. Stir until the sugar has dissolved into the cream and a smooth caramel is formed. Stir in the salt and set aside to cool. Sauce may be refrigerated for up to several weeks.