Trim the ends off the rhubarb then cut into medium slices (around 1in/1.5cm thick).
Place the rhubarb, sugar and water in a small saucepan and place over a medium heat. Warm, stirring now and then to dissolve the sugar. Bring to a simmer, reduce the heat then cook for around 7 - 10 minutes until the rhubarb becomes fork tender. Remove from the heat and let cool slightly.
Meanwhile, wash, hull and roughly chop the strawberries. Place the strawberries, rhubarb mixture and lemon juice in a blender and blend to break the fruit up well. While you don't have to, I suggest you press the mixture through a fine strainer to remove seeds and any stringy bits of rhubarb. Press through as much as you can and discard the seeds and solids.
Chill the mixture in the fridge for at least an hour (or longer, if possible - overnight is good) so the mixture is well chilled.
Following your ice cream maker's instructions, pour the chilled rhubarb-strawberry mixture into the ice cream maker and churn until it reaches a soft-serve consistency (around 20 minutes). Then, transfer to a container to freeze until firmer and for longer storage. It will likely need at least two or three hours to firm up. When you are ready to serve, you may need to let it sit a minute to soften slightly before scooping.
Notes
If you don't have an ice cream maker, you can freeze the mixture in a container and remove from the freezer to break up ice crystals roughly every 30 minutes. As it is almost frozen, if it still looks to have larger ice crystals, you could pulse in the food processor to break up and be more even texture then freeze again to firm up.This is relatively tart, as sorbets go, but it's fitting for the more tart rhubarb flavor. If you prefer it a bit more sweet, you could add up to around ⅓ of the original quantity of sugar (ie go from ¾cup/150g to 1 cup/200g for one recipe quantity).