Set your oven to 425°F and let your baking dish warm up inside it while the oven heats. A hot pan helps the batter start cooking the moment it goes in, which gives the casserole a better rise. I usually give it a light spray of oil so nothing sticks later.
Add the flour, milk, eggs, pumpkin puree, vanilla, cinnamon or pumpkin pie spice, and melted butter into a blender. Blend for about 30 seconds until the mixture looks smooth and lump-free. If you don't have a blender, a whisk and a bit of elbow grease works just fine too.
Carefully pull the preheated pan out and pour the batter in evenly. Work quickly here since the pan is hot and you want that sizzle-start effect that helps the casserole rise properly in the oven.
Slide the pan back into the oven and bake for 20 to 25 minutes. You're looking for a golden top that's puffed up around the edges, with a slight jiggle in the very center that firms up as it cools.
Let the casserole rest for about 5 minutes once it's out of the oven. It will deflate a little, which is completely normal. This short rest makes slicing much cleaner and keeps the inside from being too wet.
Cut the casserole into squares or wedges, just like you would a sheet cake. Serve warm with your favorite toppings while it's still soft and a little steamy in the middle.