Nectarine Crumble

This is from an Ina Garten recipe. The original recipe calls for plums, but nectarines are equally as delicious. Bird's custard is just as delicious as cream by the way!

12 nectarines, pitted and sliced thinly
1 1/2 cups light brown sugar
1/4 cup flour 
6 Tbs creme de cassis liqueur 
For the topping: 
1 1/2 cups flour 
3/4 cup sugar 
3/4 cup light brown sugar
1/2 tsp salt
1 cup oats
1/2 cup chopped walnuts
1/2 pound cold unsalted butter, diced

Preheat the oven to 375 degrees F. 
In a large bowl, combine the sliced nectarines, brown sugar, flour, and creme de cassis. 
Pour the fruit mixture into a 12 by 8-inch shallow baking dish. 
For the topping, combine the flour, sugar, brown sugar, salt, oats, walnuts, and butter in the bowl of an electric mixer fitted with the paddle attachment. 
Mix on low speed until the mixture is crumbly. 
Sprinkle over the nectarine and creme de cassis mixture. Bake the nectarine crumble for about 45 minutes, until the nectarines are bubbling and the top is browned. Serve warm with cream or ice cream. The topping is superb and I'm sure it would be as delicious on other kinds of fruit as well.