To prepare topping: Combine flour, sugar and cinnamon in a medium bowl.
Cut the butter into 1/2-inch squares and add to the bowl. Work with hands until the butter breaks into pea-sized pieces. Stir in walnuts and set aside. (The topping can be held for a week at this point, covered well, in the refrigerator.)
Cut nectarines into 1/4-inch thick slices, cutting over a bowl to catch the juices. Add blackberries to the bowl. Stir together the flour and sugar and mix into the fruit.
Pour into a 7 x 11-inch rectangular baking dish or gratin dish, making sure that the fruits are evenly distributed. Cover the fruit with the topping ingredients.
Place in the oven and bake approximately 45 minutes. An ideal crisp should bake until the topping starts to brown and almost blacken in some spots. Allow the crisp to cool slightly. Place a dollop of crème fraiche on each serving.