I like mine mashed: Susan Gates, The Spud From Outer Space, (London: Penguin, 2004).
Don't laugh, this is actually rather good, partially because Gates is one of a miniscule number of children's sf authors who trust her readers with two plots, one sf and one fantasy, and leaves them wondering who the good guys are supposed to be.
Cruncher, an eleven year old addicted to crisps and thumbsucking, grows his thumbnail long and sharp so he can't suck it, but can't give up his twenty a day habit of Chapel crisps. He is hoping to get rid of both bad habits before starting secondary (high) school in the summer.
Cruncher's grandad runs the council rubbish tip, a small recycling/resale business on the side, and dreams of securing the rubbish concession in space. He's built his own van for space trash reclamation.
Professor Kettles is the man who grew up making friends with potatoes and now owns the factory which makes Chapel Chips. To make the chips he has a wonderful mechanical hand called The Claw. Kettle doesn't quite see the contradiction between loving potatoes and making them into chips, but Cruncher does. Cruncher is also pretty disappointed to find out that when the bag says "made by hand" it doesn't mean by a little old lady in an apron, but by the claw. It's the first step on the road to disillusion.
Two things then happen. A potato grown in space (oddly, in Martian soil--I think Gates slipped there) lands on earth and starts growing. It is carnivorous and must be defeated.
It's chief enemy though is the ghost of the preacher Silas Smite who used his pulpit in the eighteenth century to rail against the evils of the potato (as Gates points out, while Smite didn't exist, the story is true: potatoes are related to Deadly Nightshade and are also easy to grow, thus encouraging sloth amongst the peasantry). It's protector appears to be a ghostly girl, Jane Shore, who burned down her village and like Cruncher has a long thumbnail. This turns out to be the sign of a potato eater--without knives, the poor grew their nails long to skin the potato.
Smite enslaves Cruncher and the Professor and tries to get them to kill the potato. Jane kills Smite. The claw, which has run away from the factory and has adopted Cruncher as its mummy, rescues him from a potato slurry quicksand pool but gets rejected for its pains. Grandad defeats the potato with his truck.
All ends happily as Jane Shore redeems her reputation--it was Smite who burned her village and the claw is adopted by Cruncher. And along the way we've learned a surprising amount about the potato, the manufacture of potato chips, ballistics, and space ecology.