Make the dough: In a medium bowl, combine 1/4 cup water and sugar. Stir to dissolve sugar. Add yeast, and set aside until foamy, about 5 minutes. Add remaining 1/2 cup water, oil, salt, and flour. Stir until dough forms around spoon. Turn out onto a lightly floured surface, and knead until smooth, about 5 minutes. Lightly oil a medium bowl. Place dough in bowl, turning to coat with oil. Cover with plastic wrap, and set in a warm place until doubled in size, about 1 hour.
Preheat oven to 450 degrees with rack in center. Lightly oil a 13-by-18-inch rimmed baking sheet. Make the salt wash: In a small bowl, combine water, oil, and salt; stir to dissolve salt. Roll dough into a 13-by-18-inch rectangle. Fit into pan, folding in excess to form a slightly raised edge. Prick all over with the tines of a fork. Brush salt wash evenly over dough. Set aside to rise for 30 minutes.
While dough is rising in the pan, make the filling: Tie thyme, parsley, and bay leaf together with a piece of kitchen twine to make a bouquet garni. In a large straight-sided skillet, heat oil over medium heat. Add onions, garlic, bouquet garni, and salt. Cover, and cook, stirring occasionally, for 5 minutes. Reduce heat to medium low, and continue to cook, covered, stirring occasionally until very soft and translucent, about 25 minutes more. Transfer to a sieve to drain any excess liquid. Remove and discard bouquet garni.
Spread onion filling evenly over risen dough. Place anchovies decoratively over top of tart. Bake for 12 minutes. Rotate pan, and bake until crust is cooked through and golden, about 12 minutes more. Remove from oven, and using a spatula, transfer tart to a wire rack. Arrange olives decoratively on tart. Serve warm or room temperature.