Preheat the oven to 350°F. Grease and flour an 8-inch springform cake pan.
Pour the mixure of nuts into a dry frying pan and toast them in the pan over medium heat for 8-10 minutes, stirring frequently, until the nuts take on a deeper color and start to smell nutty. Remove the nuts from the pan and set aside in a small bowl.
Melt the butter and maple syrup together in a medium (4 quart or larger) saucepan (or in your microwave), then stir in the flour, sugar, salt, and nutmeg to create a crumbly mixture. Divide this mixture in half, and carefully press one-half of the mixture into the base of the prepared springform pan.
Stir together the sour cream, baking soda, and egg, then stir this into the remaining half of the flour-butter mixture. Spoon this liquidy batter over the pressed in base.
Roughly chop the toasted nuts and sprinkle them over the batter.
Transfer the cake to the oven and bake for 40 minutes, or until the center of the cake springs back to a gentle touch and a tester comes out clean. Remove the cake from the oven and let it cool completely before carefully extracting it from the springform pan.