You can make this sweeter with more sugar, or richer with some cream instead of all milk, but there's something to be said for those rare recipes that I would describe as, 'just sweet enough.' You can also use slivered almonds instead of sliced, if you'd like.
Taste whatever fruit you're using for sweetness, since you may want to adjust the sugar level based on that. Another key, especially if you're using pears or apples, is to make sure you slice them thin; otherwise they will not cook through by the time your custard is cooked.
As I mentioned you could cook the fruit first, but I'll leave that up to you. If you use the traditional cherries, or something like tender juicy berries, this will actually cook faster than the time is given here, so I'd start checking for doneness after 25 to 30 minutes.