For Nigella's Brownies can you just the leave the walnuts out completely and not substitute them with anything?
Nigella's recipe for Brownies (from HOW TO BE A DOMESTIC GODDESS) contains walnuts as the nuts provide some crunchy contrast to the soft, fudgy brownies. However it is possible just to omit the nuts completely and bake the brownies nut-free. The baking time will be the same even without the nuts.
If you did want to use an alternative then most nuts are good substitutes and we would suggest pecans, hazelnuts, brazils or unsalted peanuts. However pistachios tend to turn a little too soft so we would avoid these. A slightly tart dried fruit, such as dried cherries or cranberries, would also be a possible alternative. Or for a decadent version add white or milk chocolate chips instead.