Line a 20cm square tinwith baking paper, leaving overhangs at opposite sides to make lifting easier.
Pre-weigh the pistachios, glacé cherries and marshmallows. Reserve about 10 glacé cherries and 1 tablespoon of pistachios for the topping. Roughly chop the reserved pistachios.
Place the white chocolate into the Thermomix bowl and grate for 8 sec/speed 9. Scrape down the sides of the bowl with a spatula.
Add the vegetable oil and melt for 3 min/50°C/speed 1 without the measuring cup. Scrape down the bowl, then melt for a further 2 min/50°C/speed 2 without the measuring cup.
Add the pistachios, glacé cherries and marshmallows. Mix for 10 sec/reverse/speed 2. Use a spatula to give the mixture a good stir so everything is evenly coated with chocolate.
Quickly transfer the mixture to the prepared tin and spread evenly. Scatter over the reserved cherries and pistachios. Refrigerate for 3 hours, or until set. Slice to serve.
Notes
White chocolate: Use white chocolate melts from the baking aisle, not cooking chocolate, as they melt and set more reliably. An exception is Lindt white chocolate, which also melts well.Substitutions: Glacé cherries can be swapped for dried cranberries or dried cherries. Pistachios can be replaced with other nuts if preferred.Optional: Pistachios can be lightly toasted in a frying pan over low heat for extra crunch.Melting temperature: Keep the temperature at 50°C when melting the white chocolate to prevent it from seizing or going grainy.Avoid moisture: Leave the measuring cup off while melting, as even a small drop of water can cause the chocolate to seize.Work quickly: Once the mix-ins are added, the chocolate will start to cool and set, so transfer the mixture to the tin promptly.Slicing: If the rocky road is firm straight from the fridge, let it sit at room temperature for 10-15 minutes or use a warm knife for cleaner cuts.Storage: Store the rocky road in the fridge in an airtight container for 1-2 weeks.