1 cup granulated sugar (100 gr)
3 eggs
½ cup freshly squeezed grapefruit juice (120 ml)
½ cup (1 stick 4 oz) unsalted butter, cut into pieces (115 gr)
1 tsp. grapefruit zest (4 gr)
To make the filling, whisk the sugar and eggs in a 4-quart nonreactive saucepan until they are light in color. Whisk in the juice and butter, and heat over medium heat until the butter is melted. Continue to whisk until the mixture thickens, 3 to 5 minutes, and coats the back of a spoon, taking care to avoid curdling the egg. Strain through a fine-meshed sieve, stir in the zest and chill. The curd will thicken as it cools. You’ll probably have more than you need; use the extra on toast, with biscuits or to fill a tartlet.