First, prepare dough for tart shell. Mix 1 cup room temperature water with yeast and sugar. Allow to sit for 5 minutes. In a large mixing bowl combine yeast mixture with 2 cups flour, salt, olive oil and egg. Stir with a wooden spoon until forms a wet, shaggy dough. Pour dough out on a well-floured surface. Begin kneading, adding the remaining 1 cup flour a few tablespoons at a time to prevent dough from sticking from your hands. Continue kneading and adding flour (as needed to prevent sticking) until dough becomes smooth and pliable, about 5 minutes. Place dough in a large bowl with ½ tablespoon of olive oil and turn dough to coat. Cover bowl with a dishtowel and set aside until doubled in size, about 1 hour.
While dough rises, prepare the filling. Cook onions with butter, salt and sugar in a large pan over medium-high heat. Stir frequently and continue cooking until onions are very tender but not brown, about 20 minutes. (Reduce heat if necessary to prevent onions from caramelizing. Onions should be soft, reduced in size by about half and should taste sweet when done.) Remove from heat and allow to cool. In a small bowl, combine cooled onions, sour cream, eggs and pepper. Mix until smooth.
Preheat oven to 350 degrees. When tart dough has finished rising, divide dough into two even parts. Roll one half out on a lightly floured surface and transfer to a 12-inch tart pan, pressing to evenly fill the bottom and sides and removing pieces that hang out over the edges of the pan. Repeat with remaining half of dough and a second 12-inch tart pan. Freeze one tart pan for later use.
Pour onion and sour cream filling into tart shell. Filling should come to the top of the shell (it will get puffy but will not expand while baking). Bake until filling is golden and firm, about 35 minutes. Allow tart to cool for at least 10 minutes before cutting into slices. Serve immediately while still warm or allow to cool completely and serve at room temperature.