If using fresh brioche, slice it into 1½-inch thick pieces and let them sit uncovered at room temperature for 2-3 hours, or dry in a 250°F oven for 10 minutes per side. This prevents soggy centers.
In a shallow baking dish or pie plate, whisk together the eggs, heavy cream, sugar, vanilla extract, cinnamon, nutmeg, and salt until completely smooth and the sugar dissolves.
Heat a large skillet or griddle over medium-low heat. Add 1 tablespoon butter and ½ tablespoon oil, swirling to coat the surface evenly.
Working with one slice at a time, dip the brioche into the custard mixture for 30-45 seconds per side. The bread should be saturated but still hold its shape when lifted.
Place the soaked bread onto the heated skillet, leaving space between slices. Cook for 4-5 minutes until the bottom is deep golden brown with crispy edges.
Flip carefully and cook the second side for 3-4 minutes. For very thick slices, reduce heat to low and cover with a lid for the final 2 minutes to ensure the custard cooks through.
Transfer to a serving plate and repeat with remaining slices, adding more butter and oil to the pan as needed between batches.
Serve immediately with warm maple syrup, a dusting of powdered sugar, and fresh berries if desired.
