Wash, pit, and slice your peaches, and place them in a large bowl. Wash, hull, and slice your strawberries into quarters- toss them into the bowl as well. Stir in the sugar and lemon juice to coat everything evenly. Cover, and place the whole mixture into fridge for at least 1 hour before serving.
Line a baking sheet with parchment paper and set aside.
In a large bowl, stir together the flour, sugar, baking powder, and salt. Using a pastry cutter or your hands, cut in the shortening until the mixture has small, pea-sized chunks of shortening throughout.
Stir in the buttermilk and vanilla until the dough begins to form. Turn the dough out onto a floured surface, and knead together gently until the dough comes together completely, being careful not to over-work it.
Pat the dough into a rectangle about 7-8 inches long and 4 or so inches wide- it doesn't have to be perfect. Cut the dough into 6 even shortcakes, and place on your lined baking sheet. Put the whole thing into the freezer for 20 minutes.
Meanwhile, preheat your oven to 400 degrees, and melt the butter to brush on top. Once the oven is preheated, pull the shortcakes out of the freezer. Brush the tops with melted butter, and sprinkle generously with the coarse sugar. Bake for 13-15 minutes, or until the tops are golden brown and the shortcakes have puffed.
In a large bowl, beat the heavy cream and sugar until stiff peaks form. If using the instant pudding mix, beat it in with the vanilla extract. You should have a firm, fluffy whipped cream.
To serve- slice a warm shortcake in half and spoon on some of the fruit mixture, including some of the juices. Add a generous dollop of whipped cream, and top with the top of the shortcake. Add more fruit and whipped cream on top if you like! Enjoy!