Make the crusts: Pulse the flour, almonds, sugar and salt in a food processor until the nuts are finely ground. Add the butter and pulse until the mixture looks like coarse meal. Add 3 tablespoons ice water and pulse until the dough starts coming together. Turn the dough out onto a piece of plastic wrap, pat into a disk and wrap in the plastic. Refrigerate until cold but still malleable, about 15 minutes. Meanwhile, preheat the oven to 375 degrees F.
Lightly butter four 4-inch tart pans with removable bottoms. Divide the prepared dough into 4 pieces, then press into the bottom and up the side of each prepared pan. Freeze until firm, about 10 minutes, then use a paring knife to trim the excess pastry.
Place the tart shells on a baking sheet, line each with foil and fill with pie weights or dried beans. Bake until golden around the edges, about 20 minutes. Remove the foil and weights and continue baking until the crusts are golden brown all over, 5 to 10 more minutes. Let cool, then carefully loosen the crusts but leave in the pans on the baking sheet.
Meanwhile, make the filling: Whisk the sugar, lemon zest, lemon juice, orange juice, cornstarch and 1/4 cup water in a medium saucepan until smooth. Whisk in the whole egg and egg yolks. Cook over medium heat, whisking, until the mixture simmers, about 5 minutes. Add the butter and limoncello and continue whisking over medium heat until thick like pudding, 4 to 6 more minutes. Spoon the curd into the prepared crusts.
Make the meringue: Beat the egg whites in a medium bowl with a mixer on medium speed until foamy, about 1 minute. Sprinkle in the cream of tartar and continue beating until soft peaks form, about 3 minutes. Gradually beat in the sugar and beat until stiff shiny peaks form, 4 to 5 more minutes. Transfer the meringue to a pastry bagfitted with a star tip and pipe onto the tarts. Bake at 375 degrees F until the meringue is set and golden, about 15 minutes. Let cool completely before unmolding.