The ingredients for making a fruit tart are milk and eggs, and you can also make a fruit tart by putting the ingredients into a cooking pot and waiting a little while.

Milk and eggs can be purchased at Sophie's Shop, and the easiest fruit to obtain is apples, which can be found in the trees outside the town, so get close enough and kick a few times to get them. The time to put in the ingredients is only ten seconds or so, if you miss the time to put in the ingredients you will fail to make it.