I start by preheating my oven to 350°F so it's fully heated by the time my batter is ready. I grease my baking dish with butter or a light coat of nonstick spray so nothing sticks once it's baked. Taking this step first means I'm not scrambling while my wet ingredients sit out.
In my large bowl, I stir together the oats, pumpkin pie spice, baking powder, and salt until they're evenly combined. I make sure there aren't any clumps of baking powder hiding in the oats. This step only takes a minute, but it keeps the spice distributed evenly throughout the bake.
In my medium bowl, I whisk the pumpkin puree, maple syrup, eggs, milk, melted butter, and vanilla until smooth. I like to whisk a little longer than feels necessary so the eggs are fully incorporated. This gives me a silky batter with no streaks of pumpkin left behind.
I pour the wet mixture into the bowl of dry ingredients and fold everything together with my spatula. I fold just until I don't see any dry oats left, since overmixing isn't really a concern here but I like to keep it simple. If I'm adding nuts or chocolate chips, I fold those in last.
I pour the batter into my prepared dish and spread it into an even layer with my spatula. I slide it into the oven and bake for 30 to 35 minutes, until the center is set and the top is lightly golden. I know it's done when a toothpick inserted in the middle comes out clean.
I let my baked oatmeal rest for about 5 minutes before slicing, since it firms up a bit as it cools. I serve mine warm with a drizzle of maple syrup or a splash of milk poured right over the top. It's the kind of breakfast that makes the whole house smell like a fall candle