Preheat the oven to 200°F. Set a wire rack inside a baking sheet.
In a large, shallow dish (a pie pan or an 8" square pan works well), whisk together the granulated sugar, cinnamon, salt, and nutmeg. Add the eggs, half-and-half, and vanilla, and whisk until combined and no streaks of egg remain.
Heat a large nonstick skillet over medium-high heat.
Working with two slices of bread at a time, soak the bread in the egg mixture for 1 minute on each side; the bread should be saturated but not soggy.
When the skillet is hot, coat the bottom with a thin slick of vegetable oil (1 to 1 ½ teaspoons) and a small pat of butter (the oil is for even browning; the butter is for flavor).
Add the soaked bread and cook, adjusting the heat as necessary, until the bottom is nicely browned, about 2 minutes. Sprinkle the bread with raw sugar, if using, then flip and cook until browned and caramelized on the other side, 1 to 2 minutes. Transfer the French toast to the wire rack and keep warm in the oven.
Repeat with the remaining bread slices, wiping out the pan between batches if using the raw sugar (to prevent burning) and greasing the pan with more oil and butter as needed.
Serve the sourdough French toast warm, topped with maple syrup, a dusting of confectioners’ sugar, and/or fresh berries as desired.