Preheat your oven to 340°F (170°C).
Grease a cake pan.
Combine the dairy-free milk and apple cider vinegar, then set aside to thicken.
In a large bowl, sift together the flour, baking powder, baking soda, and sugar.
In a separate bowl, mix the thickened milk mixture, oil, and dairy-free custard.
Gently fold the wet ingredients into the dry ingredients until just combined.
Pour the batter into the prepared cake pan and arrange the chopped rhubarb on top.
For the topping, rub the cold butter into the flour, oats, and brown sugar until it resembles coarse crumbs. Sprinkle over the cake.
Bake for 50-60 minutes, or until the cake is well-risen and golden brown.