Prepare the oven and tray Heat the oven to 350°F (175°C). Line the sheet pan with parchment. Position a rack near the middle of the oven so the bottoms do not sit too close to the lower heating element.

Combine dry and wet ingredients separately Stir the oats, flour, baking powder, cinnamon, and salt together in one bowl, breaking up any flour lumps. In the second bowl, mix the pumpkin, maple syrup, oil, and vanilla until smooth.

Make the dough and let it rest Fold the dry mixture into the pumpkin mixture just until no dry flour remains. Fold in the chopped walnuts. Leave the dough for 5 minutes so the oats can absorb some moisture. It should be thick, sticky, and scoopable, not pourable.

Portion and flatten Divide into eight equal mounds, about 40 g each or a heaped 2 tablespoons. Place them at least 1 inch apart. With the back of a slightly damp spoon, flatten each to roughly 2 1/2 inches across and 1/2 inch thick, smoothing ragged edges.

Bake until set Bake for about 18–22 minutes, checking at 18 minutes. The tops should lose their wet shine, the edges should look set, and the bottoms should be lightly golden; lift one edge with a spatula to check the underside. If a cookie is very thick, it may need longer; do not remove the batch with visibly wet dough in the centers.

Cool before lifting Leave the cookies on the tray for 5 minutes, then move them carefully to a rack for about 10 minutes before serving. The centers should be soft but cohesive. Cool completely before packing or storing, allowing more time if they are still warm.