For the crust: Preheat the oven to 425 degrees. Place a baking stone or an inverted baking sheet on the center rack to heat. Use a pastry brush to paint 1 tablespoon of the oil into the corners, up the sides, and across the bottom of the slab pie baking sheet (either a 9- by 13-inch baking pan, or a quarter sheet pan).
Grate the onion on the medium holes of a box grater or use a food processor's grating disk. Place the onions in a medium mixing bowl. Grate the potatoes on the large holes of the box grater or with the food processor. Take a handful of shredded potatoes, squeeze them over a small bowl to capture the liquid, and place the potato shreds in the bowl with the onions. Continue until all the potatoes have been grated and their liquid squeezed out. Work quickly to keep the potatoes from turning pink and brown.
Let the potato liquid sit for about 5 minutes, until the starch and liquid separate. Pour off the liquid, keeping the starchy white paste at the bottom of the bowl. That's potato starch. Add the starch to the grated onions and potatoes, then add the egg and egg white, salt and pepper. Mix well with your hands. Firmly press the potato mixture into the prepared pan. Brush the surface of the potatoes with the remaining 1 tablespoon oil. Bake (on top of the stone or the inverted baking sheet if using) for 45 minutes, until toasted brown at the edges and across the surface. Remove pan from oven.
For the filling: Heat the oven to broil. Wearing rubber gloves, halve the poblanos lengthwise and remove and discard the stem, core and seeds. Place the peppers, skin side up, on a parchment-lined baking sheet. Broil until the peels blister and blacken, 8 to 12 minutes. Remove the pan from the oven and cover the peppers with a kitchen towel to steam them. (Throw that towel in the wash afterward, to avoid transferring chile heat.) As soon as it's possible to handle the peppers, wearing gloves, slip off the blistered peel and slice the peppers into thin strips to make about 2 cups.
Set the oven temperature to 400 degrees. Split the casings on the sausage, empty the sausage meat into a large skillet, and discard the casings. Turn the heat to medium and break the meat apart using 2 wooden spoons. Cook low and slow until the meat has crumbled and is no longer pink, 7 or 8 minutes. Pour off and discard as much of the fat as possible.