For dough, blend together warm water and yeast and allow to stand 5 minutes until a froth develops. Add salt and flours and mix with a wooden spoon until dough comes away from the sides of the bowl. Turn dough out onto a lightly floured surface and knead for 2 minutes, until dough has an elastic texture. Cover and allow to rise 20 minutes in a warm area.
To make tarte, pre-heat oven to 220°C and line a baking tray with parchment paper.
Cover bacon with cold water in a small saucepan; bring to a boil and drain. This step takes out extra salt and fat from the bacon and allows it to crisp up on the tarte without burning.
Cut dough into 4 equal portions and dust lightly with flour. Roll as thinly as possible into a rectangle about the size of a legal sheet (11” x 14”) and place on prepared baking tray. Spread ¼ of the sour cream over the surface of the dough. Top with ¼ each of the bacon and sliced onions. Season with salt and pepper.
Bake in the oven 8 minutes then slide the parchment off the cookie sheet directly onto the oven rack and cook 3 minutes further, until the crust is gold brown and crispy.
Slide tart onto a cutting board and cut in half lengthwise, then into 6 widthwise, to equal 12 small portions.
Make the remaining three while you snack on the first one!