Preheat the oven to 350F.

In a large nonstick skillet, heat ¼ cup of olive oil over medium heat. Add the onion and leek and cook for 8 to 10 minutes, stirring occasionally, until the onion and leek are soft and fragrant. Transfer to a large bowl.

Add the spinach to the same pan (no need to add oil) and sauté for 3-4 minutes over medium heat, stirring occasionally, until the spinach is wilted and loses its volume. Transfer the cooked spinach to a colander and cool completely.

Squeeze the water from the cooked spinach, working one handful at a time, and add the spinach to the bowl with the onion. Add the feta, ricotta, scallions, dill, eggs, salt, pepper, and nutmeg, and mix well until fully incorporated.  

Sprinkle the breadcrumbs evenly on the bottom of a 9x13-inch glass or metal baking pan or a 12-inch round baking pan.

On a clean work surface, unroll the phyllo sheets and cover them with a slightly damp kitchen towel. 

Lay 2 phyllo sheets on the bottom of the pan over the breadcrumbs, letting the edges hang over the sides of the pan, and brush with the remaining olive oil. Place one phyllo sheet so that half lies in the pan and half hangs over the edge. Repeat on the other three sides of the pan so that the 4 sheets overlap in the middle, brushing with olive oil. Lay 2 more sheets in the center of the pan for 8 layers of phyllo sheets in total, and brushing the top with olive oil. Spread the spinach filling evenly inside the pan. 

Fold all the phyllo sheets hanging outside the pan over the filling; they should overlap completely. Brush with more olive oil. Crinkle each of the 6 remaining phyllo sheets like an accordion and lay them on top, one next to the other, to create a wrinkled cover pattern. Brush with the remaining olive oil.

Before baking, use a large knife to cut through the top layers of the spanakopita just until you reach the filling. If using a rectangular pan, cut into 15-20 squares; if using a round pan, cut into 8-16 wedges. Precutting makes the pie much easier to serve, as phyllo dough becomes crisp and crumbly after baking.

Bake uncovered for 50-60 minutes, until the pie is golden brown. Let rest for 10 minutes before cutting and serving. 

Leftover spanakopita can be stored in an airtight container in the refrigerator for up to 4 days. To reheat, warm in the oven at 325F for 10-15 minutes.