Grate the russet potatoes and yellow onion, then soak in cold salted water for 20–30 minutes.

Drain the mixture thoroughly and squeeze out any remaining moisture with a clean towel.

In a bowl, combine the drained mixture with beaten egg, matzo meal, kosher salt, and ground black pepper.

Heat vegetable oil in a skillet over medium-high heat until shimmering.

Drop spoonfuls of latke batter into hot oil and shape into 3-inch rounds, frying for 6–10 minutes per side until golden brown.

Poach eggs in simmering water with a splash of vinegar for 4½ to 5 minutes.

Blend egg yolks, lemon juice, and a pinch of salt, then drizzle in melted butter until thickened.

Assemble by layering a crispy latke with smoked salmon, a poached egg, and hollandaise; top with chives and optional red onion.