Do cooked or processed frozen vegetables need a hechsher?

Answer: Yes.

Reason: If frozen vegetables were cooked or processed, don’t eat them without a hechsher.

Source:

• cRc: frozen vegetables containing added ingredients such as color, flavor, oil, pasta, sauce or spices require hashgachah, as do frozen artichoke hearts, asparagus, broccoli, Brussels sprouts, potatoes, spinach, sweet potatoes and winter squash, and those packed in Israel.
• OK: if additives are included (like preservatives, sauces or seasonings), a reliable kosher certification is required.

Related questions:

• Do frozen edamame beans need a hechsher? No, if not processed.
• Does frozen raw ful (fava beans) need a hechsher? No, if not processed. (Products from Israel are a separate question.)
• Do canned fruits and vegetables need a hechsher? Yes (OK).

More from Rabbi Tahan: Q&A: Hechsher on Frozen Corn

References: cRc (askcrc.org), “Vegetables, frozen” · OK, Consumer FAQ