To prepare the filling, heat the olive oil in a sauté pan over medium heat.

Add the chopped onion and salt. Cook until the onions become translucent, about 5 minutes.

Add the diced mushrooms and tomato paste and cook for another 3 minutes, stirring to mix the tomato paste, until the mushrooms are almost cooked.

Gradually add the spinach to the pan until wilted. After all, the spinach has wilted; set the pan aside to cool.

While the filling is cooling, prepare the beef mixture by mixing everything in a medium bowl. Don’t be afraid to use your hands in this process. After it’s all mixed, it needs to feel smooth on your hand. If it doesn’t feel right, add more bread crumbs until it does.

Spread a big piece of plastic wrap on your countertop and spread the beef mixture to make a square shape about 1/4 inch in thickness.

Place the cooled spinach mixture in the middle of the beef mixture, but make sure to not spread it over it because we will roll the beef mixture up to make a spinach centre.

This is the tricky part. Using a plastic wrap, roll the beef mixture horizontally to meet two ends. Seal the ends using your hands. Place it on a small baking dish seam side down. Remove the plastic wrap slowly, ensuring you keep the ends of the roll together.

Now you can preheat your oven to 400 degrees Fahrenheit.

Cover and place the dish in the refrigerator for 10 minutes.

Whisk the egg white to break the lumps, brush the top with it, and put it in the oven for 30 minutes.

You may serve this spinach and mushroom beef roll with plain greek yogurt on the side.