Grate the russet potatoes and onion. Soak in salted water for 20-30 minutes, then drain and squeeze out moisture.

Combine squeezed potato and onion mixture with beaten egg, matzo meal, salt, and pepper. Mix until well incorporated.

Heat vegetable oil in a skillet. Form latke mixture into rounds and fry for 6-10 minutes on each side until golden brown.

In a saucepan, bring water to a simmer, add vinegar, and poach eggs for 4.5-5 minutes until whites are set and yolks are runny.

Melt unsalted butter, whisk egg yolks and lemon juice together, and slowly drizzle in butter. Whisk until desired consistency.

Assemble by placing a latke on a plate, topping with smoked salmon (if using), poached egg, and hollandaise. Garnish and serve.