Grease an 8 inch square baking dish with the butter.
In a large bowl, toss the chopped rhubarb with 1 tablespoon maple syrup, the orange rind and juice.
Tumble into the baking dish and spread out evenly.
In another large bowl, mix the oats, ¼ cup of the chopped almonds, the baking powder, cinnamon, ginger, nutmeg and salt.
Pour over the rhubarb mixture in the baking dish and spread over the top evenly.
In a medium bowl, whisk together the milk, eggs, remaining ⅓ cup maple syrup and the almond flavouring until light and frothy.
Pour over the oat mixture evenly, pressing gently with the back of a spoon to encourage the liquid to sink down into the rhubarb.
Cover and refrigerate overnight or for at least 6 hours.
In the morning, preheat the oven to 350°F (175°C).
Take the oatmeal casserole out of the fridge while the oven preheats.
Stir the casserole very gently to mix, pressing it back down with the back of a spoon to settle it before baking.
Sprinkle the top of the casserole with the remaining ¼ cup chopped almonds.
Bake for 40 to 50 minutes until golden brown and set.
Serve in bowls with a little yogurt drizzled over top if desired. You can also garnish with fruit.
Leftovers will keep, covered, in the fridge for up to 2 days.
Reheat gently before serving.