He was a precise cook but not an adventurous one; his skills didn’t extended beyond jam and methamphetamine. He’d learned the recipes around the same time. The meth was a business, lifestyle and illness - the jam was more of a hobby.
He never left the house, just got high and made jam and sold meth. He thought it was a ruse: whenever anyone came to buy meth they left with a jar of jam. He joked he was a jam dealer.
Meth addicts don’t really get hungry, so no-one cared (or even knew) that the jam was inedibly sweet.