Prepare the crust. Dissolve the yeast and sugar in 2 tablespoons warm water in a small bowl. Let stand for 6 to 8 minutes: the mixture should foam like a head of beer.

Combine the yeast mixture, olive oil, flour, and salt in a food processor fitted with a chopping blade or a mixer with a dough hook. Pulse the machine to mix these ingredients. Add the egg white and water and run the processor in bursts until the ingredients come together to form a dough that is soft but not too sticky. Add a little water if the mixture seems too dry. Knead the dough in the food processor for 2 to 3 minutes or by hand on a lightly floured work surface for 5 minutes, or until smooth. You can also make the dough in a heavy-duty mixer fitted with a dough hook.

Transfer the dough to a lightly oiled bowl and cover with plastic wrap. Let the dough rise in a warm, draft-free spot until doubled in bulk, about 1 hour. Punch down the dough and refrigerate it until using. The dough can be made the day before.

Meanwhile, prepare the filling: Heat the olive oil in a large nonstick frying pan or sauté pan. Add the onion and 2/3 of the minced garlic and cook over medium heat until soft and lightly browned, about 4 minutes.

Reduce the heat slightly and stir in the rosemary, cabbage, and potatoes. (Depending on the size of the pan, you may need to add the cabbage gradually.) Cook the mixture, covered, for 15 minutes, stirring occasionally. Uncover the pan and continue cooking the vegetables for 5 to 10 minutes, or until they are very tender and all the pan juices have evaporated, stirring often. Stir in the flour.

Stir in the ricotta. Cook the mixture until thick and creamy, about 2 minutes. Remove the pan from the heat, let cool for 2 minutes, and stir in the egg white. Correct the seasoning, adding salt, pepper, and cayenne to taste: The mixture should be highly seasoned. Remove the pan from the heat and let cool. Stir in the remaining 1 clove garlic.

Roll out 1/2 of the dough and use it to line a 7 x 11-inch tart pan or baking dish, or a 9-inch springform pan, lightly oiled with spray oil. Let at least 1 inch of dough hang over the edge. Arrange all of the potatoes in a layer on the bottom. Spoon the cabbage mixture on top. Crumble the goat cheese over the cabbage. Brush the top of the overhanging dough with a little of the egg-white glaze.

Roll out the remaining dough. Use most of the dough to form a cover for the tourte, leaving enough for a 1-inch overlap. Place the cover over the filling and pleat the top and bottom edges to form a tight seal. Brush the top with egg-white glaze. Cut any leftover dough into fanciful shapes, like leaves and berries, and use them to decorate the top of the tourte. Brush these shapes with glaze. Preheat the oven to 350° F.

Using a sharp knife or razor blade, cut 5 or 6 slits in the top crust to allow any steam to escape. Bake the tourte on the bottom rack of the oven for 40 to 50 minutes, or until the crust is a rich golden brown.

Transfer the tourte to a cake rack to cool for 5 minutes. Unmold the tourte and cut into wedges or rectangles for serving.