Preheat oven to 375°F. Line a 12 cup muffin tin with papers.
Stir all of the topping ingredients together in a small bowl and set aside.
In a large mixing bowl, stir together the flour, brown sugar, baking powder, salt and nutmeg. In a separate, smaller bowl, stir together the milk, oil and vanilla. Pour the milk mixture into the bowl with the flour mixture and stir just until blended, being careful not to overmix. Carefully fold in the raspberries, rhubarb and nuts.
Divide the mixture into the lined muffin cups. The cups should be very full, but not overflowing. Sprinkle the tops with the oat topping.
Bake for 22-24 minutes, or until the muffins are lightly browned on the tops and a toothpick inserted into the center of a muffin comes out clean.
Transfer the muffins to a cooling rack and allow to cool completely before removing from tins.