Prepare the potatoes: Peel the uncooked potatoes and coarsely grate. You should have about 3 cups of shredded potato. Put the potatoes in a bowl and cover them with cold water to retain color and moisture. Drop in a few ice cubes. Let the potatoes soak while preparing the desired fillings.
Prepare fillings, chopping as needed to be an appropriate size for a filling. Set aside. Note: Fillings should be at room temperature or warmer.
Drain the potatoes. Spread the shreds on a tray lined with paper towels to absorb excess water, and pat dry with more paper towels. Transfer to a large mixing bowl. Finely grate onion and squeeze out excess moisture. Gently mix onion with the potatoes to thoroughly combine.
Lightly oil an 8- to 10-inch skillet. For best results, use a non-stick or very well seasoned cast iron skillet. Sprinkle enough potato mixture to cover the skillet with a very thin layer. Use a spatula to press down into a flat layer, covering the surface of the pan. Cook over medium-low heat until golden brown on the bottom, about 4 or 5 minutes. Watch carefully to prevent scorching.
Use spatula to loosen edges. Fully cover the top of the skillet with a sheet of parchment paper, then place a large plate or baking tray on top. Flip the skillet over to invert the potato omelet onto the plate or tray; parchment paper will be on the bottom. If necessary, lightly re-oil the skillet, then slide the potatoes back in, with the cooked side up. Press down with the spatula and cook until golden brown on the underside, about another 3 to 4 minutes.
Assemble the omelet by sliding the potato “jacket” onto a serving plate, and placing fillings as desired down the center. Fold up the sides, envelope style. (Or, place desired fillings in the center while potatoes are still in the pan.) Fold one side up to cover the fillings. Flip the omelet over so the seam side is down. Cook for another minute or so to seal the omelet and heat through, then slide onto a serving plate. Top with Sriracha Cream to add a kick of spice to your omelet.
Sriracha Cream: Combine sour cream with sriracha sauce and whisk until smooth.
Drizzle cream over omelets as liberally as you like.