Combine 1 cup (240 ml) water, brown sugar, and mushrooms in a small saucepan. Bring to a boil over medium-high heat, then immediately reduce the heat to low and simmer gently, covered to help avoid over-reducing the syrup, for about 5 minutes. Remove from the heat and let stand, covered, for 10 to 15 minutes to cool slightly, then blend until smooth using a blender or immersion blender.
Combine the oats, almonds, walnuts, sunflower seeds, and pumpkin seeds in a large mixing bowl. Pour in the candy cap syrup and the oil and stir until all of the ingredients are evenly coated.
Heat the oven to 300°F (150oC). Line a rimmed baking sheet with parchment paper. Pour the mixture onto the prepared sheet and spread it into a layer about ¼ inch (6 mm) thick. Transfer to the oven and bake for 1 to 1½ hours, until evenly and lightly browned, stirring every 10 to 15 minutes to ensure even browning.
Remove from the oven and let cool completely, then mix in the dried fruit.
