Preheat the oven to 180°C (160°C fan/Gas Mark 4) and line a 20 cm square baking tin with baking paper, leaving an overhang.
Place 100 g coconut oil (or unsalted butter) and 100 g runny honey in a medium saucepan and gently heat on a low hob until the oil has melted and the mixture is smooth and combined (do not boil).
Remove the pan from the heat and stir in 250 g rolled oats and a pinch of salt until every flake is coated.
If using, add 50 g mixed seeds (e.g. sunflower or pumpkin) and/or juicy raisins, then mix thoroughly.
Tip the mixture into the prepared tin and press down firmly and evenly using the back of a spoon or a spatula.
Bake for 15–20 minutes, or until the edges are lightly golden (avoid overbaking to prevent hardness).
Remove from the oven and allow the flapjacks to cool completely in the tin—they will firm up as they cool.
Once cool, lift the slab out using the baking-paper overhang and cut into 12 squares or bars.
Store in an airtight container at room temperature for up to one week.