Preheat the oven to 350°F (175°C).

Tear the croissants into large pieces and place them in a greased casserole dish.

In a large bowl, whisk together the eggs, milk, heavy cream, melted butter, sugar, vanilla extract, ground cinnamon, and fine sea salt until well combined.

Pour the egg mixture over the croissant pieces, pressing them down to soak.

Sprinkle the blueberries or mixed berries evenly over the top.

Let the casserole sit for about 15 minutes to soak.

Bake in the preheated oven for 45 minutes, or until golden brown and set.

Serve warm, garnished with additional berries if desired.