Preheat oven to 300 degrees. Combine all ingredients in a large bowl until well coated, then spread mixture on a rimmed baking sheet, patting into an even 1/4-inch-thick layer.
Bake 35 minutes. Use a wide spatula to flip in large sections. Flatten back into a single layer and bake 10 minutes more. Let cool completely on a wire rack.
Scrape up baked seed mixture with spatula and break into chunks. Clusters can be stored at room temperature up to 1 week.