Preheat the oven to 325 F. Lightly butter an 8-inch square baking pan and set aside.
Process dates in a food processor until they begin to gather together in a ball, about 1 minute.
In a small heavy-bottom pan on the stovetop, warm the maple syrup, almond butter and orange juice over medium heat. Whisk well so the almond butter fully incorporates into the maple mixture. Slowly pour into the bowl of the food processor, add the vanilla extract, and process for another minute or so, or until the dates loosen into the warm maple mixture (should look like a really thick nut butter at this point).
In a medium bowl, whisk together your muesli, sesame seeds, cinnamon, and salt. Scrape date mixture into the bowl of muesli along with the orange zest and flax or millet and stir until all the grains and nuts are coated (while it gets a little messy, I use my hands at this point). Try to work relatively quickly so as not to let the mixture cool too much.
Turn the mixture out into the prepared pan and press firmly so it covers the surface evenly. I use the back of a spatula here to help. Bake for 25-28 minutes (see note below), or until the edges of the bars are just turning slightly brown. Let cool for at least 30 minutes, and ideally 1 hour, to allow bars to fully set. Slice into bars the size of your choosing and serve room temperature. Cover and store leftovers at room temperature for up to one week.
A quick note on baking the bars: It can be difficult to tell when the bars are done. Don’t wait until they’re uniformly golden or dark brown on the top, or until they’re completely dry or firm to the touch. They will be perfect if they’re just turning lightly brown around the edges but should still give way to your touch in the center — much like cookies when they come out of the oven, the bars will firm up as they cool. Don’t be tempted to cut corners and slice them before they’re cool, however, or they won’t hold together well for you. Give them at least 30 minutes and preferably 1 hour to cool and set completely. Enjoy!