Praise be to Allaah.
Allaah says (interpretation of the meaning):
“He it is Who created for you all that is on earth…” [al-Baqarah 2:29]
“O mankind! Eat of that which is lawful and good on the earth…” [al-Baqarah 2:168]
“Say: ‘Who has forbidden the adornment with clothes given by Allaah, which He has produced for His slaves, and al-Tayyibaat [all kinds of halaal (lawful) things] of food?’…” [al-A’raaf 7:32]
From this, we know that everything that grows from the earth is in principle halaal (permitted), and no-one should decree it to be haraam (forbidden) or makrooh (disliked) without sound Islamic evidence (daleel). What evidence is there that things that grow beneath the ground, like potatoes, carrots and onions, etc., are makrooh? What is the strange principle on the basis of which this lady – about whose background we know nothing – states that cooking a number of “makrooh” things in one pot results in haraam food?
If the resulting dish was intoxicating or poisonous, or was cooked with something haraam such as pork, then we would say that it is haraam to eat it. But the cooking of five or six kinds of root vegetables together, as you describe, is something permissible, and there is nothing wrong with it. We advise you not to give any weight to the words of those who have no knowledge. Allaah is the One Whom we ask to guide us to the truth and to help us understand our religion. May Allaah bless our Prophet Muhammad.