1) Heat the oil in a heavy large frying pan over medium heat. Add the pancetta and saute until crisp and golden, about 5 mins. Using a slotted spoon, transfer the pancetta to a bowl.
2) Add the onion to the pan drippings in the same pan and saute until translucent, about 4 mins. Add the spinach and garlic. Saute over medium-low heat until the garlic is tender, about 2 mins. Stir in 1/2 tsp of salt, 1/4 tsp of pepper, nutmeg, and the cooked pancetta.
3) Place half of the bread cubes in a buttered 3 litre baking dish. Sprinkle half of the cheese over the bread, then top with half of the spinach mixture. Repeat layering.
4) Whisk the milk, eggs, remaining 1/2 tsp of salt and 1/4 tsp of pepper in a large bowl and pour evenly over the strata. Chill the strata, covered with clingfilm, at least 2 hrs and up to 12 hrs.
5) Preheat the oven to 180°C/ gas mark 4.
6) Bake strata uncovered until puffed, golden brown, and cooked through, 40 mins.