Recipe

Make Ahead: The filling will keep for up to one week in the fridge, covered in plastic wrap pressed directly into the surface. The dough can be made up to 5 days in advance, wrapped well, in the fridge, or up to one month in the freezer.

Make Dough

1. In food processor, combine flour, almond flour or sliced almonds, sugar and sea salt; pulse to mix if using almond flour, or process until nuts are finely ground if using sliced almonds.
2. Add butter and pulse just a few times to break pieces into chickpea-size chunks. Drizzle in beaten egg, then pulse until dough comes together. If it seems very crumbly, add a little water, 1 tbsp at a time, until it just holds together.
3. Pat into a 1″-thick disk and wrap in plastic. Chill in fridge for at least 1 hour.

Prepare Filling

1. Grate zest of 5 lemons and reserve. Juice zested lemons to make 1 cup of juice; if you’re a bit short, use the 6th lemon.
2. In large heatproof bowl, whisk eggs well and set aside.
3. In medium pot over medium heat, stir together lemon zest, lemon juice, sugar, butter, oil and sea salt.
4. Once mixture is simmering and butter has melted, slowly pour or ladle about a third of mixture into eggs, whisking constantly, to temper them. Then return the egg-lemon mixture to pot.
5. Reduce heat to medium-low and cook, stirring constantly with a heatproof spatula or wooden spoon, until filling thickens enough to coat back of spoon, about 5 minutes. Strain into medium bowl and set aside to cool.

Assemble & Bake Tart

1. Roll out dough on lightly floured surface to form 13″ round, about 1⁄4″ thick. Line 10″ tart pan with dough, trim edges and prick all over with fork. Chill shell, uncovered, for 30 minutes.
2. Heat oven to 350°F. Line tart shell with foil and baking weights (or dried beans or raw rice), and bake for 15 minutes. Then, remove foil and weights and bake until crust is golden brown, another 5 to 10 minutes. Transfer tart pan to wire rack and let crust cool slightly, about 5 minutes.
3. Reduce oven temperature to 325°F. Scrape filling into tart shell, smoothing top. Bake until filling is set around edges and jiggles only slightly in center when pan is shaken, 45 to 55 minutes. The filling may brown slightly and that’s OK, but cover edges of tart with foil if crust starts to get too brown.
4. Transfer tart to wire rack and let cool completely. Before serving, sprinkle top with fleur de sel, if you like. Serve with crème fraîche, if desired.