Melt the butter in a large skillet over medium heat. Add the onion and season with salt and pepper. Cook, stirring occasionally, until the onions have caramelized and softened completely, about 15 minutes. Remove from the heat and set aside.

Soak the matzo in a large bowl of warm water for a few seconds to soften. Drain the matzo in a colander.

Beat the eggs in a large bowl, and season with salt and pepper. Add the soaked matzo with a spatula stir to combine. Let the mixture sit for 2-3 minutes.

Return the skillet to medium-low heat, add the matzo mixture to the caramelized onions, and season again with salt and pepper. Cook, scraping the bottom of the skillet occasionally, almost like you’re making a soft scramble. Cook until the eggs are just set, about 3-5 minutes, then remove the skillet from the heat.

Transfer the matzo brei to a large bowl (or serve straight from the skillet) and serve with plenty of sour cream and applesauce. Store matzo brei in an airtight container in the refrigerator for 2 days.