Prepare pastry by combining flour, salt and sugar in a food processor. Add butter and combine until it has a coarse crumb consistency. With the machine running, add the water. Form into a ball. Roll out into a circle large enough to fit into a 10-inch tart pan, about ? inch thick.
Preheat oven to 400°F.
Brush mustard over the bottom of the pastry shell. Top mustard with mozzarella slices.
Beginning at outer edge of shell, make a layered overlapping row of the tomato slices. Make a second row and then fill the center with a final circle of tomatoes.
Sprinkle top evenly with garlic and oregano. Season to taste with salt and pepper and drizzle the olive oil over all.
Bake for about 40 minutes.