Begin by making the rhubarb. Arrange the rhubarb in a snug, even layer in a pan, and scatter over the sugar. Pour over the water and add any additional flavourings. Very gently poach the rhubarb for 10-12 minutes, until the sugar is dissolved and the rhubarb is beginning to release some of its colour. Using a slotted spoon, lift the rhubarb into a smaller container where it can fit snugly, then allow the poaching liquid to cool to room temperature and pour over the cooked rhubarb. Transfer to the fridge to chill.
Next, make the granola. In a small saucepan, combine the butter, sugar, vanilla and salt and stir gently until the sugar is completely dissolved.
Pour the oats, nuts and seeds into a large mixing bowl and pour over the buttery mixture, stirring to coat everything evenly.
Divide between two large baking trays and cook in the oven for 20 minutes, then remove and give everything a good stir. Cook for a further25 minutes, stirring once more during this time, until the granola is deep golden and the kitchen smells like a bakery. Turn the oven off, stir once more and return both trays to the oven to cool.
Allow the granola to cool completely before decanting everything into an airtight container.
Serve the granola with yogurt and some of the poached rhubarb spooned over the top.