Prep the pan: Line your baking pan with parchment paper, leaving some overhang on the sides.
Warm and mix: Add the nut butter and honey to a large bowl. Microwave for 15 to 20 seconds, until smooth and easy to stir.
Add the oats: Add the oats, ground flax seeds, cinnamon, and salt. Stir well until everything is evenly coated. If the mixture feels a bit dry or hard to mix, add 1 to 2 tablespoons of milk. Fold in your mix-ins.
Press and chill: Transfer the mixture to the pan. Press it down very firmly, especially in the corners. Refrigerate for at least 2 hours, or freeze for 30 minutes, until firm. Lift out, slice into bars, and store in the freezer.
