In large bowl, stir together cereal oats, flax and chia seeds; set aside.
In small saucepan heat peanut butter and honey over medium heat, stirring often, until melted and smooth. Remove from heat and stir in vanilla. Pour over cereal mixture and stir to combine well.
Transfer to a parchment lined 10x15 baking dish. Evenly press mixture to form a dense even layer, about 1-inch thick. Sprinkle with a bit of sea salt (optional).
Allow to cool and set. Cut into squares.
Notes
Store in airtight container at room temp for a few days. In fridge it will last up to a few weeks.