Melt butter and toss carrots and onions until coated. Cover and cook gently for 10 minutes, adding a little water if necessary. Add boiling water or broth, and rice. Cover and cook for 20 minutes.
Whizz in the food processor or put through a food mill until smooth. Return the purée to pan and reheat, tasting for salt and pepper. Add butter, divide among serving bowls and top with chervi