Preheat your oven to 375°F (190°C). Generously butter a 9-inch round baking dish or cast-iron skillet.

Arrange the sliced nectarines, cut-side up, uniformly in the bottom of the prepared dish. Sprinkle lightly with 1 tablespoon of the granulated sugar.

In a large bowl, whisk together the flour, remaining granulated sugar, and salt. In a separate bowl, whisk the eggs until slightly frothy, then whisk in the milk, melted butter, vanilla extract, and almond extract (if using).

Gradually pour the wet ingredients into the dry ingredients, whisking gently until just combined. Do not overmix; a few small lumps are acceptable. The batter should be thin.

Gently pour the batter over the nectarines in the baking dish, ensuring the fruit remains relatively covered. Let the batter rest for 5 minutes before baking.

Bake for 40 to 45 minutes, or until the clafoutis is puffed, golden brown, and a knife inserted near the center comes out clean. The center will deflate slightly upon cooling.

Let cool on a wire rack for at least 15 minutes before serving. Dust generously with powdered sugar before slicing.