In a 9-inch cast-iron pan, heat 2 tablespoons of the olive oil over medium heat until the oil is hot, but not smoking. Add the chopped onions, spreading them evenly over the bottom of the pan. Decrease the heat to medium-low and let cook, undisturbed, for approximately 10 minutes.

Sprinkle the onions with ½ teaspoon of the salt and cook, stirring occasionally, until browned and broken down, 30 to 45 minutes. Once golden and caramelized, transfer the onions to a large bowl.

Preheat the oven to 350°F.

Add the remaining 3 tablespoons of olive oil to the cast-iron pan and place it in the oven to heat up while you prepare the potatoes.

Fill a large bowl with ice water.

Using a food processor fitted with the shredding disk, or a box grater on the largest hole, grate the potatoes. The potatoes will oxidize, so be sure to shred right before use.

Add the potatoes to the bowl of ice water. Let sit for 10 minutes to remove excess starch.

Drain the potatoes, transfer them to a clean kitchen towel, and wring out any excess liquid. The more liquid you can remove, the better! Add the potatoes to the bowl with the caramelized onions.

Add the remaining 1 teaspoon of salt, eggs, black pepper, and matzo meal. Stir to combine.

Carefully remove the cast-iron pan from the oven and spread the potato mixture in the pan, pushing it down to compact the potatoes. It should sizzle on contact with the pan. Return the pan to the oven and bake for 1 hour, or until deep golden brown on top.

Serve with sour cream and chopped chives.