Cook and peel the potatoes in a pot with water (you will need to reserve 150 ML of the water). Once the potatoes are cooled, pass them through a ricer, into a large mixing bowl.
Meanwhile proof the yeast with some of the reserved WARM potato water (about 1/4 cup) and 11/2 tsp sugar. Let stand for 5 minutes until foamy.
Add the flour, and mix until crumbly. Add the yeast mixture, the melted margarine and slowly add the remaining potato water. At this point you can either mix by hand or put it in your Kitchen Aid mixer with the bread hook attachment. Be careful when mixing the water, because the dough consistency must not be too dry or too wet, it should be tacky to the touch and come together into ball. Keep kneading until a smooth ball forms.
Place the dough in a well greased bowl, and cover it with seran wrap or a towel. Place in a warm spot to rise -- I usually put it in my oven to avoid drafts. Let is rise for about one hour or until doubled in size.
While the dough is rising assemble the filling. Mix all the ingredients together and set aside.
Grease a 12 inch sping form pan. Se aside.
Preheat oven to 350 degrees.
Once the dough doubles, take it out of the bowl and gently deflate it and divide the dough in half. Place the first half in the mold covering th eentire bottom. Put the filling on top of the dough but leave 1/4 inch border so as the filling does not touch the sides. Place the second half of the dough on top of the filling.
Put the sliced tomatoes on top of the second half of the dough, sprinkle with salt and drizzle with olive oil. Bake for about 40 minutes or until done.