The phrase "to overcook the vegetables" means that the vegetables have been cooked for too long, causing them to become soft, mushy, or lose their natural flavors and nutrients. Full definition
Don't overcook the vegetables as they will continue cooking in the sauce later. (phamfatale.com)
I can imagine overcooking the vegetables like that could be a problem. (101cookbooks.com)
Known as passive cooking, this on - and - off technique not only saves energy but can also help avoid overcooking vegetables such as corn on the cob. (scientificamerican.com)