Adjust the oven shelf to the middle position. Preheat the oven to 350°F/180°C. 

Line the base of a 14x9x1.5-inch (35x24x4-cm) baking tin with baking paper. 

Toss the cake mix, All-Bran flakes, oats, sunflower seeds and nuts together in a large mixing bowl. Melt the butter in the microwave in a 1-litre glass measuring jug. Add the oil and buttermilk and mix well. Beat in the eggs. Pour over the dry mixture in the bowl and (using a spatula) stir everything together until well blended. Do not overmix. 

Scoop the mixture into the prepared baking tin and smooth down to level. Bake for 45 minutes. While still warm, cut the rusks into singles. Freeze them overnight (still in the baking tin). 

The following day, remove the rusks from the baking tin and arrange them on a metal cooling rack or baking sheet (leaving space between each rusk). Return them to the oven and bake at 250°F/120°C, allowing them to dry out for 2–3 hours. Leave to cool down completely before storing in an airtight container.