To make the overnight oats, blend or whisk the non-dairy milk with the dates or syrup (use a blender if you're using dates for sweetness) and the vanilla extract. Transfer the oats and chia seeds to a glasslock or other airtight storage container and pour the milk over them. Stir everything well. Cover and place in the fridge overnight. In the morning, add an extra splash of almond milk if the mixture is very thick.
To prepare the stewed rhubarb, place the rhubarb, water, and sugar in a small saucepan. Brig it to a boil as you stir, to dissolve the sugar. Reduce the heat to a simmer and simmer, uncovered, for 15-20 minutes, or until it has thickened to resemble a loose/runny jam. Taste and add an extra pinch of sugar if it's a little too tart for you.
To serve, scoop the overnight oats into two bowls and top each with some spoonfuls of the stewed rhubarb. Leftover stewed rhubarb will keep for a week in an airtight container in the fridge, and it can be used on toast or stirred into traditional hot breakfast cereal.