1) Grease a large gratin dish or casserole with 5g of the butter.
2) In a large bowl, whisk together the eggs, milk, cream, nutmeg, cayenne, salt, and pepper. Set aside. In a small bowl, toss together the ham, tomatoes, parsley, and chives.
3) To make the strata: Scatter a 50g of the cheese over the bottom of the buttered dish, cover with one-third of the bread, and then half of the tomato mixture. Repeat. Top with a final layer of bread and remaining cheese.
4) Pour the egg mixture over the strata and gently press it down with the back of a large spoon to make sure the top layer is moistened with the custard. Melt the remaining 15g of butter and drizzle it over the strata. Cover with plastic wrap and refrigerate overnight.
5) Preheat the oven to 180C/Gas 4. Bake the strata, uncovered, until golden brown and slightly puffed, about 45 to 55 minutes. Let rest 10 minutes before serving.