1. To make the dough, in a large mixing bowl, stir together the flour, sugar, and salt. Cut the butter into cubes and mix together with your hands or using a pastry blender until it’s in small pieces no larger than the size of corn kernels.
2. Add one egg and the water, and mix until the dough holds together. Roll the dough out on a lightly floured surface until it’s about 18-inches (45 cm) round.
Brush off any excess flour and fit into a 9- or 10-inch high-sided round baking dish or cake pan; the edges will overhang the sides quite a bit. (If using a springform pan, double wrap the outsides with foil to avoid leaks.)
3. Beat the remaining egg in a small bowl with a fork, then brush the insides of the dough with the egg.
4. Prepare the filling by slicing the apples into eighths. Mix them together with the figs, grapes, 1/3 cup (65 g) sugar, and Armagnac, then transfer the filling into the tart dough. Strew the walnuts over the top of the fruit.
5. In a small bowl, mix together the crème fraîche with the egg and pour it over the fruit and nuts. Lift the edges of the dough and cover the fruit, then sprinkle a good amount additional sugar over the top of the dough.
(When making tarts like this, I brush the dough with melted butter to help it stick.)
6. Put the tart on a baking sheet and bake in a 425Fº (218ºC) oven for 55 minutes to 1 hour, until the top of the dough is browned and the fruit is cooked through, which you can verify by poking the center with a paring knife; when done, it should meet no resistance.
(Note: During baking, the walnuts may darken, as shown. This is a rustic touch and typical. If you are concerned, you can drape the tart with foil if they become too dark for your taste.)
7. Remove the tart from the oven and let cool down a bit before serving.