Trim and chop the rhubarb, then place in a large pan with 175g of the caster sugar and 100ml of water, and cover with a lid.
Bring to the boil, then remove the lid and simmer for 5 minutes, until the rhubarb is cooked. Set aside to cool.
Spoon into a blender and blitz to a purée. Squeeze in the juice of half the lime. Taste and add more sugar and lime if needed (it should have a subtle, not overpowering, flavour).
Churn in an ice cream maker for about 20 minutes, or until smooth and set. If you don’t have an ice cream maker, put the mixture in a tub and place in the freezer. Every hour give it a good stir. Repeat until your sorbet is frozen to the right consistency. Transfer to a nice dish and store in the freezer.
To make the brittle, use vegetable oil to lightly grease a tray lined with baking paper.
Mix the matcha powder with a little hot water, then top it up to 75ml. Pour into a small pan with the remaining caster sugar, and place on a medium-high heat.
Bring it to the boil, but don’t stir – just gently swirl it. Simmer gently for 4 to 5 minutes, or until it turns a deep brownish-green colour (or the temperature reaches 150ºC on a sugar thermometer, the ’hard crack’ stage), then pour into the greased baking tray.
Roughly chop and scatter on the pistachios and 1 teaspoon of sea salt, then leave to cool. Once set, break into pieces.
To serve, let the sorbet stand at room temperature for about 10 minutes, then scoop into bowls and sprinkle with shards of the brittle.