Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang.
In a medium saucepan over low heat, combine the coconut oil or butter and honey. Stir gently until melted and smooth—do not let it boil.
Remove from heat and stir in the rolled oats and a pinch of salt until all the oats are coated.
If using, add the mixed seeds and/or raisins, then mix until they’re evenly distributed.
Transfer the mixture to the prepared pan and press it down firmly and evenly with the back of a spoon or a spatula.
Bake for 15 to 20 minutes, or until the edges turn lightly golden—avoid overbaking to keep the bars chewy.
Let the pan cool completely on a wire rack; the bars will firm up as they cool.
Once cool, lift the slab out using the parchment overhang and cut into 12 squares.
Store in an airtight container at room temperature for up to one week.