Prepare your bread by slicing the brioche into 1-inch thick slices. If your bread is very fresh, you can leave the slices out for 30 minutes to dry slightly, which helps them absorb the custard mixture without falling apart.
Make the custard mixture by whisking together eggs, milk, heavy cream, sugar, vanilla, cinnamon, nutmeg, and salt in a large, shallow bowl until well combined and slightly frothy.
Soak the bread slices in the custard mixture, allowing each slice to absorb the liquid for about 30 seconds per side. Don’t oversoak or the bread may become too soggy and fall apart during cooking.
Gently press blueberries into both sides of the soaked bread slices, about 6-8 berries per slice, pressing them in just enough to stick without tearing the bread.
Heat your skillet or griddle over medium heat and add 1 tablespoon of butter, swirling to coat the pan surface completely.
Cook the French toast for 3-4 minutes on the first side until golden brown, being careful when flipping as the blueberries can make the bread slightly more delicate.
Flip carefully and cook for another 2-3 minutes on the second side until golden and the center is cooked through but still moist.
Add fresh butter to the pan between batches for the best flavor and to prevent sticking.
Keep cooked slices warm in a 200°F oven while you finish cooking the remaining bread.
Serve immediately with warm maple syrup, a dusting of powdered sugar, and additional fresh blueberries if desired.