Preheat the oven to 300°F. Line a rimmed baking sheet with a wire rack and set aside.
In a large shallow bowl, whisk the eggs, milk, sugar, cinnamon, nutmeg, and vanilla until well combined.
Using a sharp serrated knife, cut the bread into 1‑ to 1 1/2‑inch slices on a slight diagonal.
Melt the butter in a large skillet over medium-high heat.
Dip each side of the bread into the custard mixture, first on one side, then turning to the other side and pressing gently into the custard to fully soak the bread. Let excess custard drip back into the bowl before transferring to the skillet.
Working in batches of 2 to 4 slices, add the soaked bread pieces to the skillet, being careful not to crowd the pan. Cook until the bottoms are deeply golden, 2 to 3 minutes. Flip and cook until the second side is golden and the centers feel set when pressed, another 1 to 2 minutes.
Transfer the cooked French toast to the prepared baking sheet and keep warm in the oven while you cook the remaining slices. They should not brown much further in the oven.
Arrange the French toast slices in a single layer on the baking sheet or on serving plates. Sprinkle the tops evenly with the powdered or superfine sugar, using just enough to lightly cover the surface.
Using a kitchen torch, move the flame in slow, sweeping motions over the sugared tops until the sugar melts, bubbles, and turns a deep golden brown, 1 to 3 minutes. Hold the tip of the flame a couple of inches from the surface and keep it moving to avoid scorching any one spot.
Serve immediately while the topping is still crisp, with sliced strawberries on the side or scattered over the top, if desired.