If using fresh rather than day-old bread, spread the cubes on a baking sheet and toast in a 275°F (135°C) oven for 10-15 minutes until slightly dry but not browned.
In a large bowl, whisk eggs until frothy (about 30 seconds). Add milk, cream, sugar, vanilla, cinnamon, nutmeg, and salt. Continue whisking until thoroughly combined and sugar dissolves.
Butter a 9x13-inch baking dish generously. Arrange half the bread cubes in an even layer.
If using mix-ins, sprinkle half over the bread. Pour half the custard mixture evenly over the bread, ensuring all pieces are moistened.
Repeat with remaining bread, mix-ins, and custard. Press down gently with a spatula to help bread absorb the liquid.
Cover with plastic wrap, pressing it directly onto the surface. Refrigerate for at least 4 hours or overnight (preferably 12-24 hours).
Remove the strata from the refrigerator 30 minutes before baking. Preheat your oven to 350°F (175°C).
Remove the plastic wrap, drizzle melted butter over the top, and sprinkle with brown sugar.
Bake uncovered for 45-55 minutes until puffed, set in the center, and golden brown on top. A knife inserted in the center should come out clean.
If the top browns too quickly, tent loosely with foil after 30 minutes of baking.
Allow to rest for 10 minutes before serving for the best texture and easier slicing.