Gently toss the sliced apple with the Marsala and set aside, stirring occasionally.
Whisk the ricotta, ¼ cup (50 g) of the sugar, grated chocolate, egg, egg yolk, lemon zest and nutmeg together. Strain the Marsala from the apples into this ricotta mixture and stir to blend. Pour the ricotta filling into the baked tart shell and arrange the apples nicely overtop. Brush the apples with the melted butter and sprinkle them with the remaining one tablespoon (12 g) of sugar.
Bake the tart for about 25 minutes, until the apples are tender when pierced with a fork. Cool the tart to room temperature then chill until ready to serve.
The tart will keep, refrigerated for up to two days.