Preheat oven to 400°F.
Remove the pits from the peaches and cut into slices roughly 1-inch at the thickest part. In a medium mixing bowl, gently stir the slices with the sugar and lemon juice and set aside while assembling the biscuit topping.
Combine buttermilk and salt in a measuring cup and stir well to combine. Place in the freezer until ready to use. Set aside 2 tablespoons of the butter to melt and brush over the tops of the biscuits.
Place flour in a large mixing bowl. Add the baking powder and whisk to combine, then sift together three times.
Chop remaining butter into 1/4 inch cubes. Dump the flour out onto a clean work surface. Toss the butter cubes in the flour. Use a rolling pin to roll the butter into the flour, stopping to scoop everything back into a pile and rolling back out a few times, until there are lots of smears of butter and some of the flour is starting to look a little like wet sand.
Scoop the flour and butter back into the mixing bowl. Add the buttermilk mixture all at once. Use a large fork to gently push the buttermilk and flour around until they are combined. Scoop the dough out onto the work surface again and knead once or twice, just until it is a cohesive dough. Use a rolling pin to gently pat and roll the dough flat. Tap or roll out to about 3/4 inch thick.
Cut biscuit dough into rounds with a biscuit cutter or flat-sided glass. Gently push scraps back together and cut, until one biscuit worth of scraps remains. Use hands to shape that one gently into a round.
Place the peach slices along with their sugar and juices in a Shallow Round Oven (or 3 1/2 quart Dutch Oven or other pot). Arrange the berries over the top. Arrange all the biscuits on top of the fruit to cover.
Place the pot, uncovered, into preheated oven. Bake for about 30 minutes, until the fruit juices are bubbling, and the biscuits are starting to brown on top. Remove the pot from the oven and brush the tops of the biscuits with the melted butter and sprinkle with turbinado sugar. Place the Dutch Oven back in the oven and bake for about 3 more minutes, until biscuits are golden brown on top.
Remove and let cool until still warm but not hot. If taking the cobbler somewhere, simply place the lid over the top for easy portability and storing.
To serve, spoon cobbler into bowls. Top with a scoop of cold ice cream and nasturtium petals, if desired.