Preheat the oven to 350°F. In a saucepan over medium heat, combine 1 cup of the sugar with the lemon juice and 1 tablespoon water and cook, stirring constantly, until the sugar dissolves and turns caramel in color, 8 to 10 minutes. Carefully divide among 8 (5- to 6-ounce) ramekins.

In the bowl of a stand mixer fitted with the whisk attachment, combine the remaining 1/2 cup sugar and the eggs and whisk until pale and thick, about 4 minutes. Add the orange juice and triple sec and whisk to combine.

Pour the egg mixture into the ramekins over the just-set caramelized sugar. Transfer the ramekins to a large roasting pan and add enough hot water to come halfway up the sides of the ramekins. Cover the pan with aluminum foil. Bake until the top jiggles but is set, about 40 minutes. Let cool for 15 minutes, then refrigerate for at least 8 hours or up to overnight.

To serve, run a knife along the flan edges to loosen, dip the bottoms of the ramekins in hot water for 20 seconds, then unmold by inverting each one over a clean dessert plate, garnish with orange zest, and serve.