Preheat oven to 300°F/150°C and coat a 9x9-inch pan with cooking spray.
Finely chop the cashews and dried fruit - a food processor works well for this. Then add them to a large bowl, along with the pepitas, sunflower seeds, oats or sesame seeds, flaxseed, and salt. Stir to combine.
In a separate bowl, microwave the honey until warm, just a few seconds. Drizzle honey and vanilla over the seed mixture. Add the nut butter, if using, and stir to combine. Dump the mixture into the prepared pan and spread evenly. Spray a sheet of waxed paper with cooking spray, then place on top of the mixture in the pan, sprayed side down. Use your hands to gently pack down the mixture. Remove the waxed paper.
Bake until golden, about 20 minutes. Cool completely.
Place a cutting board over the top of the pan, then flip it all over to transfer the bars from the pan and onto the cutting board. Cut into 16 bars.
Wrap the bars individually in plastic wrap or snack bags, and store in the refrigerator for up to 1 month.