Thoroughly butter an 8 x 12inch roasting pan and place it in the freezer for 5 to 10 minutes. Combine the brown and granulated sugars and the water in a heavy stainlesssteel saucepan over medium heat and, stirring occasionally, cook until the sugars have dissolved and the mixture is simmering. Increase the heat to high and, without stirring, simmer the syrup until it reaches 300 degrees on a candy thermometer (hardcrack stage). Remove from the heat and add the baking soda and the nuts, mixing thoroughly to make sure they are evenly blended in. Pour the mixture onto the buttered slab and allow to cool completely. Lift off and break the brittle into bitesized pieces. The candy will keep in an airtight glass jar for several weeks.

Note: To toast nuts, preheat the oven to 350 degrees. Spread the mixed nuts on a cookie sheet and toast in the oven until golden, about 15 minutes. Set aside to cool.