How to Make It
MAKE THE FLATBREAD: In a standing electric mixer fitted with the paddle, mix the yeast and sugar into the water until dissolved. Let stand until foamy, about 10 minutes. Mix in the powdered milk and the 1 1/2 tablespoons of olive oil. In a medium bowl, mix the flour with the salt. Add the flour to the yeast mixture and mix on low speed until thoroughly combined. Replace the paddle with the bread hook. Knead the dough at medium speed until elastic but still sticky, 10 minutes. Scrape the dough into an oiled bowl and brush the top with oil. Cover with plastic and let stand in a cool place until doubled in bulk, 1 1/2 hours.
Punch down the dough. On a lightly floured work surface, shape it into a ball and transfer to an 11-by-16-inch baking sheet. Brush the top with oil, cover with a damp kitchen towel and let rest for 30 minutes. Using lightly oiled fingers, press the dough out to the edge of the baking sheet. Brush lightly with oil, cover with the damp towel and let stand until puffed, 1 1/2 hours.
MAKE THE TOPPING: In a large skillet, bring 1/4 inch of water to a boil. Add the clams, cover and cook over moderately high heat until they open, 5 to 8 minutes; discard any that don't open. Remove the clams from the shells and put in a bowl.
Set a pizza stone on the bottom of the oven and preheat to 500°. Wipe out the skillet, add the bacon and cook over moderately high heat until browned; using a slotted spoon, transfer to paper towels to drain. Add the garlic to the skillet and cook until fragrant. Add the onion and cook, stirring, until golden, about 6 minutes. Add the thyme, remove from the heat and stir in the bacon. Season with salt and pepper.
In a medium skillet, cook the potatoes in the olive oil over moderately low heat until just tender, 6 minutes.
Stretch the dough out to the corners of the baking sheet. Lightly brush with oil from the potatoes. Spread the onion mixture over the dough and top with the clams. Arrange the potato slices on top and season with salt and pepper. Bake the flatbread on the heated pizza stone for 12 minutes, until the crust is golden. Sprinkle with parsley, cut into triangles and serve.