Preheat the oven to 180°C (350°F). Place the chia seeds and water in a small bowl. Give them a quick stir and leave to soak for about 10 minutes, until a thick gel has formed.
Peel and de-stone the mango. You'll need 1 cups worth of chunks and then you can cut the remaining mango into chunks or slices, as desired. I like to chop some of the remaining mango into small chunks to stir through the oatmeal mixture and leave the rest for the top. Place 1 cup of mango chunks into a blender along with the almond milk. Blitz until smooth.
In a large bowl, combine the oats, almonds and salt. Add the mango and milk mixture, the oil and the vanilla extract along with the chia gel. Mix well. Stir in the small chunks of mango, if liked.
Grease a large baking dish with oil. Pour the oat mixture into the dish and spread out evenly.
Cut the rhubarb into short lengths and place into a bowl along with the reserved mango chunks or slices. Add the honey and toss to coat. Place the fruit on top of the oatmeal. Bake for 45 minutes until set and golden brown on top.