Whisk together the milk, heavy cream, eggs, sugar, vanilla, cinnamon, and salt in a large bowl. Pour this into a large shallow dish, like a 9x13 baking pan.
1 cup milk, ⅓ cup heavy cream, 5 large eggs, 1½ tbsp granulated sugar, 1½ tsp vanilla extract, 1½ tsp cinnamon, ½ tsp salt