Preheat your oven to 325°F (165°C). Generously butter a loaf pan to prevent sticking.

Melt the butter in a skillet over medium heat until it becomes bubbling.

Add the diced onion to the skillet and cook for 1 minute, stirring occasionally. Once done, turn off the heat and let it cool.

In a large bowl, place the drained salmon and mash it with a fork until it is flaky.

Add the crushed saltine cracker crumbs, rice wine vinegar, kosher salt, Worcestershire sauce, cayenne pepper, and the cooled onion mixture. Mix everything until smooth.

Separate the eggs, adding the yolks to the salmon mixture and placing the whites into a clean bowl.

Stir the egg yolks into the salmon mixture until well combined.

Whisk the egg whites to medium stiff peaks and gently fold them into the salmon mixture in two additions.

Transfer the salmon mixture into your prepared loaf pan and smooth the top with the back of a spoon.

Bake in the preheated oven for about 45 minutes, or until a toothpick inserted into the center comes out clean.

Let the salmon loaf rest for 5 minutes before carefully inverting it onto a cutting board. Slice and serve warm.