Preheat the oven to 300 degrees F. Line two rimmed baking sheets with parchment paper.

In a large bowl, combine the oats and walnuts. Measure the oil into a 2-cup measuring cup, making sure to swirl it around the walls of the cup before pouring the oil over the oats and nuts.

Using the same measuring cup, combine the maple syrup, brown sugar, salt, cinnamon, nutmeg, ginger, and cardamom. Whisk until the sugar has mostly dissolved into the syrup, and the spices are well-incorporated.

Pour the syrup slurry over the oats and nuts and stir until well-coats.

Divide the oat and nut mixture between the two prepared pans and arrange the nascent granola into an even layer. Use the back of your spatula to press the oats and nuts down a little. This will help the formation of clusters upon cooling.

Bake the trays for 40-45 minutes, taking care to switch their positions in the oven at least once.

When the granola has taken on a golden hue, it is done. Remove the pans from the oven and let the granola cool completed before touching it.

Once the trays are cool enough to handle, grasp the edges of the parchment paper to gently break the granola into fat clusters.

Store in air-tight bags, containers or jars.