Grease a 9x13-inch ceramic baking dish generously with softened butter. Arrange half of the cubed brioche bread evenly across the bottom, scatter with 1 cup of fresh blueberries, then top with the remaining brioche cubes and blueberries.

In a large mixing bowl or 4-cup measuring pitcher, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla bean paste, cinnamon, nutmeg, and sea salt until completely smooth and cohesive.

Slowly and evenly pour the custard mixture over the bread cubes, pressing down gently with a spatula so every piece absorbs the liquid. Cover tightly with plastic wrap and refrigerate overnight (or at least 4 to 8 hours).

In the morning, remove the baking dish from the refrigerator and let it sit at room temperature for 25 to 30 minutes while preheating your oven to 350°F (175°C).

Prepare the streusel: In a medium bowl, combine flour, brown sugar, rolled oats, and cinnamon. Cut in cold cubed butter with a pastry blender or your fingertips until crumbly pea-sized clusters form. Scatter evenly across the soaked french toast.

Bake uncovered at 350°F (175°C) for 45 to 50 minutes until puffed, golden brown, the edges are caramelized, and a knife inserted in the center comes out clean. If the top browns too quickly, tent lightly with foil during the last 10 minutes.

Let rest for 10 minutes before slicing. Dust generously with powdered sugar and serve warm with cascades of pure maple syrup!