(*If you're using leftover oatmeal, omit these first two steps.)Put oats in a large bowl and cover with one or two inches of water. Add yogurt and mix in. Cover and soak overnight at room temperature. Drain oats and rinse. Make sure as much of the water is drained off as possible - bounce the oats gently in the colander to make sure. Return to bowl.
Preheat your oven to 375°F/190°C.
Beat the 6 eggs with the 1 cup of milk, ½ cup of syrup, 1tsp. of vanilla, 1 tsp. each of spice, and ½ tsp salt. Add to oat bowl.
Mix in the remaining ingredients except the sliced butter.
Pour mixture into a 9 x 13 buttered casserole dish. Scatter sliced butter around the top - this is something McGruther recommends in Nourished Kitchen for her recipe, and it just makes the finished product magical. Of course, you'll add more butter on top once its baked.
Bake for 45 minutes or until the sides are golden and the center is slightly smooshy. Let it cool for five minutes before serving warm with maple pecan butter (see post).