Dissolve the yeast in 3 tablespoons warm water; stir in sugar and set aside until foamy, about 5 minutes.
Place flour and oil in a large bowl or stand mixer. Add the dissolved yeast and about 200ml water, or as much as needed to make a sticky dough. Transfer onto a floured surface; add salt and knead for 8 to 10 minutes or until smooth and soft. Or you can fit the stand mixer with the hook attachment; add salt and work for the same time or until the dough sticks around the hook.
Lightly oil a large bowl and place the dough in the bowl; cover with a clean, slightly damp drying cloth and place in a cold oven with light on. Let rise for 2 hours or until doubled in size.
Punch down the dough and split in 2 equal parts. Cover and let rise for another hour.
Cook the spinach in a pan with 2 tablespoons of water until wilted, about 7 to 8 minutes. Transfer in a colander and squeeze out as much water as possible.
Use a fork to mash the ricotta in a bowl; add cooked spinach, grated Parmesan, salt, pepper and nutmeg.
Grease a 30cm round tin with plenty of olive oil. Place one piece of dough on the tin and stretch it out with your hands to cover the base, leaving a 3cm rim. Turn the dough to coat it in the oil. Spread the tomato passata on top of the dough; cover with mozzarella and spoon on the spinach and ricotta mixture.
Roll out the second piece of dough so it is wide enough to sit on top of first piece; secure it on top by pinching the edge to seal. Cover the loaf with a clean, slightly damp drying cloth and set aside to rise for 20 minutes.
Preheat the oven to 220 C / Gas 8. Brush the top of the focaccia with the oil and water mixture and prick the top of the loaf with a fork.
Bake in the preheated oven for 20 minutes or until it has puffed up and is puffy golden on top. Serve warm.