Preheat oven to 500 degrees F. In a large cast-iron skillet, toss potato slices with water, olive oil, salt, and pepper. Tightly cover skillet with aluminum foil. Bake about 10 or 15 minutes, until the potatoes are soft but not browned. Transfer the potatoes to a plate.
Prepare the broiler, skillet or baking stone. Roll out pizza dough.
Measure and have all toppings ready. Layer potato rounds on dough, leaving 1 inch uncovered around the edge of the crust. Dollop ricotta cheese on the potatoes.
Broil pizza for 5 to 8 minutes, until crust has puffed around the edges and blackened in spots. Remove the pizza with a wooden or metal peel or baking sheet. Transfer to a cutting board. Sprinkle the trout, preserved lemon, and fennel slices and fronds on pizza. Let pizza stand for a few minutes.
In a small bowl whisk together yogurt and milk; drizzle on the pizza. Cut the pizza in quarters to serve.