Preheat oven to 350 degrees. Spray a medium casserole pan with olive oil.
In a medium bowl mix together oats, quinoa, walnuts, cranberries, chia seeds, baking powder, cinnamon and sea salt. Spread mixture evenly in prepared pan.
If using flax egg mix together 1 tablespoon ground flax seeds with 3 tablespoons warm water. Set aside to thicken.
In a separate bowl mix together (flax) egg, maple syrup, pumpkin puree, milk, coconut oil and vanilla extract. Pour mixture over quinoa oat mixture ensuring that oats are fully covered.
Bake for 35 minutes.
Meanwhile, make streusel topping by combining flour, oats, walnuts, brown sugar and butter in a small bowl.
Remove casserole from oven and top with streusel topping. Return to oven and bake for 15 minutes longer. Serve warm with an additional pat of butter or drizzle of maple syrup if desired.