Lightly whisk the eggs in a bowl large enough to also hold the milk and whipping cream. Pour the milk and cream into a saucepan and bring to a simmer. Add the minced garlic. Slowly pour the warm milk into the eggs, whisking continuously. Add the salt, pepper, and thyme. Sauté the onions in the 1 Tbsp. (15 mL) of butter until just softened. Remove from the heat and reserve. Butter an 8- x 11-inch (20- x 28-cm) casserole dish. Cover the bottom with a layer of slightly overlapping bread slices. Sprinkle the onions and ham, if using, over the bread and top with another layer of bread. Lay the Camembert pieces on next. Cut the remaining bread into 1-inch (2.5-cm) pieces and cover the cheese with them.
Stir the egg mixture so that the garlic rises from the bottom, and pour it evenly over the strata. Cover with plastic wrap and press down so that all the bread soaks up the liquid. Leave for at least half an hour.
Preheat the oven to 375°F (190°C).
Bake for 50 minutes to 1 hour. The top should be golden. If it starts to get too dark, cover with foil. Remove from the oven and let sit 10 minutes before serving.