Peel apples (if desired) and dice into small cubes, approximately pea-sized. Slice rhubarb stalks in half lengthwise, and dice them as well. Dice the overripe strawberries, if using.
In a medium pan, add the diced fruits, 2 Tbsp flour, 2 Tbsp sugar, 1 tsp cinnamon, ½ tsp nutmeg, 1 Tbsp jam, and ¼ cup water. Stir together, then turn on the burner to medium-low heat.
Allow the mixture to heat through and fruit to soften, stirring frequently to prevent burning. If mixture is too thick or close to burning add additional water by the Tbsp. Note that the fruit will release some liquid, so be patient and don't add too much water. I usually cook mine for approximately 10 minutes, but you may need more or less time depending on the size of your fruit. They don't have to be complete mush because you want them to hold their shape and they'll be baked a bit more in the oven.
Take the pre-baked shortbread crust and pour the cooled, but soft ganache over the crust. If it has hardened too much, pop it in the microwave for 15 second increments until softened again. Spread out the chocolate in an even layer across the shortbread, reserving some for a drizzle topping as desired.
Spread the apple rhubarb filling over top of the ganache and spread it evenly across the tart.
Bake for an additional 10 minutes or until crust is toasty and just barely brown.
Remove from the oven and allow to cool for at least 30 minutes to ensure everything sets and is firm enough to cut. Slice into bars or slices, and serve warm! This apple rhubarb chocolate tart goes well with some vanilla ice cream, or a dollop of fresh whipped cream.