My De Buyer Mineral B Element iron fry pan burns everything I cook, even though I’ve been using low to medium heat with non-stick foods (mostly potatoes) and canola oil. The pan has been seasoned with grapeseed oil. I’ve tried varying the amount of oil and potato slice sizes to no success. No matter what, once they are added to the pan, the underside of the potatoes will burn and blacken within seconds.
Then you aren't using "low to medium heat". The heat is defined by how quickly your food cooks, not by the setting on the stove. Lower your heat until the food fries at a reasonable rate. As for the oil, if in doubt, err on the side of adding a bit more than you think you'll need.