Underdo

Sorry to say, I've underdone the pot roast. I think we'd better put it back in the oven for a bit.

Sorry to say, I've underdone the downspout repair. I think we'd better hire someone to replace the whole thing.

Near-synonyms: (of tasks) half-ass, shortchange, phone in

We warned him not to underdo it. This customer expects attention to detail.

Near-synonyms: underachieve, slack off