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Melt the butter in a saucepan over medium heat. Add the potato and cook, stirring often, for about 5 minutes. Add the apple and continue cooking until the apples are tender, about 5 minutes more. Add the spinach and mix until the spinach has wilted. Season with salt and pepper. Transfer the mixture to a shallow bowl and set aside in the refrigerator until needed.
3
In the meantime, heat the oil in a second saucepan over medium heat. Add the onions and cook until soft, about 8 to 10 minutes. Transfer the mixture to a shallow bowl and set aside in the refrigerator until needed.
4
Roll the puff pastry to a size large enough to cover a 7.4" tart pan and leave some extra for folding in. Trim the excess dough. Fold the perimeter over to create a double thickness of dough at the sides of the tart pan.
5
Brush the base of the dough with the mustard. Spread the spinach mixture over the base and press down lightly. Cut the sausage into 3/8" thick slices. Arrange the slices in concentric circles on top of the spinach mixture. Strew the onions over the sausage and pat down lightly.
6
Bake the tart for 20 to 30 minutes until the pastry has puffed up and is a nice color.
7
Unmold the tart shortly after it is removed from the oven.
Prep time 
