Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with a mixture of the melted butter and olive oil.

In a large mixing bowl, thoroughly combine the squeezed dry spinach, crumbled feta, ricotta, beaten eggs, nutmeg, salt, and black pepper until the mixture is uniform.

Unroll the thawed phyllo sheets and keep them covered with a damp kitchen towel to prevent them from drying out as you assemble.

Lay one sheet of phyllo into the prepared baking dish, letting the excess hang over the sides, and brush it lightly with the butter-oil mixture.

Repeat this process with 7 more sheets, brushing each layer with the fat mixture, to create a sturdy base of 8 sheets.

Spread the spinach and cheese filling evenly over the phyllo base, then fold the overhanging edges of the phyllo dough back over the filling to seal the sides.

Layer the remaining 8 phyllo sheets on top of the filling, brushing each sheet with the butter-oil mixture as you stack them.

Use a sharp knife to gently score the top layers of phyllo into 6 square portions, making sure not to cut all the way through to the bottom.

Sprinkle sesame seeds over the top and bake for 35 to 40 minutes until the pastry is puffed and deeply golden brown. Let the burek cool for 10 minutes before slicing all the way through and serving warm with a side of cold Greek yogurt.