Melt the butter in a 10 inch cast iron skillet over medium heat until it browns. Let it cool. Pour into a bowl and chill.
To the same cast iron skillet, add the nectarines and raspberries. Add the cornstarch, date sugar and salt to it and give it a toss. Preheat the oven to 375F.
In a medium sized bowl, pour the oatmeal, almond meal, flax meal, cinnamon, cardamom and date sugar. Add the chilled butter and mix well with your hands.
Spread this crumble over the fruits mix in the cast iron skillet. Bake in the oven for 30-35 mins, till the top of the crisp changes color slightly. Serve warm with vanilla ice-cream.