Start by making the custard. Put the vinegar into a small pan with the shallot , peppercorns and bay leaf and simmer until the vinegar is reduced to about 4 tablespoons. Let it cool.
Whisk together the eggs and the creams, strain in the cooled vinegar and gently stir it all together. Grate over a little lemon zest, being careful not to add any white pith, then stir in the lemon juice and mustard. Season with 1 tsp each of salt and pepper.
Lightly butter an ovenproof dish 28cmx28cmx8cm approx. Split the muffins and roughly tear each piece into quarters. If they are not stale toss the torn bread on a baking sheet and bake at 160 C 140 Fan Gas 3 for 10 minutes, then remove from the oven and leave to cool completely.
Put the pieces of muffin, spinach and ham into the oven dish and mix it all together with clean hands so that everything is evenly distributed. Slowly add the egg mixture, pouring it evenly over and making sure it reaches every corner.
Gently press with the back of a spoon. Cover with foil and refrigerate for at least 4 hours but ideally overnight.
Remove the strata from the fridge 30 minutes before you want to cook it and spoon over any of the egg mixture that hasn’t soaked in to the bread. Put the dish on a baking dish to catch any drips.
Preheat the oven to 180 C, 160 Fan, Gas 4. Scatter over the cheese. Bake, still covered with the foil, for 30 minutes, then remove the foil and bake for a further 20-30 minutes until the top has crisped up a little and is turning golden.