Peel (if desired) and coarsely grate the Russet Potatoes into a large bowl, then grate the Yellow Onion into the same bowl so the onion juices mix with the potatoes.
Place the grated potato and onion in cold water and swish to remove excess starch; drain and repeat until the water runs clearer, then transfer the mixture to a clean kitchen towel or cheesecloth and squeeze out as much moisture as possible.
Return the dried potato-onion mixture to the bowl and season with Kosher Salt and Ground Black Pepper, tossing gently to combine.
Heat a large nonstick or cast-iron skillet over medium-high heat and add a blend of Vegetable Oil and Unsalted Butter; let the butter melt and begin to foam but not burn.
Add the potato mixture to the skillet, spreading it into an even layer (or form into patties if you prefer). Press down lightly with a spatula to compact.
Cook undisturbed until the bottom is deeply golden and crispy, about 5–7 minutes, then flip sections or each patty and cook the other side until browned and cooked through, about 4–6 more minutes. Reduce heat if the exterior is browning too quickly.
Transfer the finished hash browns to a paper-towel-lined plate to drain briefly, season to taste with a little extra salt and pepper if desired, and serve immediately.
