1. In the bowl of a stand mixer, combine the buttermilk, oil and egg. Mix using the wire whisk attachment until fully combined.
2. In a large bowl, sift together the flour, sugar, baking powder, and salt. With the stand mixer running on its lowest speed, add a little a time until fully incorportated, scraping down the sides of the bowl as needed. Add the raspberries and walnuts and mix briefly to combine.
3. Pour the batter into a buttered loaf pan and bake for 60-70 minutes, until a knife inserted in the center comes out cleanly.
4. Let cool before slicing and serving.