Preheat the oven to 180C/gas 4.
Begin by making the crumble topping, as this needs some time to rest in the freezer or fridge while you prepare the rhubarb. Add the flour and diced butter to a food processor and pulse until combined. If you don’t have a food processor, the ingredients can be mixed (literally crumbled) by hand in a large bowl.
Once the butter and flour has an ideal crumble texture, gently incorporate the porridge and almond by pulsing just once or twice to avoid grounding the almonds or oats.
Put the crumble topping into a zip lock bag or bowl and chill in the freezer for 10 minutes, or in the fridge for at least 20 minutes.
Wash the rhubarb, remove the ends and slice each stick into approx. 2cm-1 inch pieces.
Add the rhubarb to a suitable baking dish for the crumble. Top with the granulated sugar, ginger, vanilla, cinnamon, a pinch of salt, the lemon juice, and a tablespoon of water. Mix and place in the oven for 15 minutes, stirring once while cooking.
As rhubarb gives off a lot of moisture, you may want to drain some of the liquid at the point, to prevent the crumble topping from getting soaked. Reserve it to use later.
Pour your cold crumble topping over the rhubarb, making sure it’s all covered, but try to keep the topping fairly loose.
Place the crumble tray in the oven for around 30-45 minutes, using a baking tray as an extra support to collect any juices that escape.
Once golden, remove from the oven, rest for five minutes and serve with custard (or cream, if you really must). Optionally drizzle some of the excess rhubarb juice from cooking over the crumble.