As Liuzhou continues to point out (very correctly), unless you are secretly hiding vegetables in some food that a child ordinarily eats and likes, I think you are barking up the wrong tree here, even if you go the Heston route.
Heston doesn't really try to fool people at the taste level usually - he primarily likes to disguise the outward appearance so they are 'surprised' when they bite into what looks like one thing and tastes like another. He may add 'smells' wafting near the plate as well to convince people that what they are about to eat is really something else - but, even that doesn't fool educated tastebuds once the food hits the palate. He may investigate the chemical composition of various foods and ingredients to further help with the 'surprise' element - but the 'surprise' element is what he is mostly about - and you are asking about making stuff that is 'no surprise' for the diner (in other words, they expect a French fry made of potato - they get something that both looks like and tastes like a French fry and leave the table believing that is what they ate - where is the 'fun' or purpose in that?) which seems quite the opposite thing. If the adult diner wants French fries made of potatoes - give them exactly that or you will fail in your stated mission in most cases I think. If they hate parsnips, why would you force an adult to eat them by trickery - and never have them know they ate them? Heston wants people to know what they ate - but only at the moment he wants them to discover that fact (at the moment they taste it).
If you are trying to fool an adult eater into thinking he just ate French fries made of potatoes, even if they look like the normally consumed item in colour, shape and presumably texture, only potatoes will probably taste like potatoes once you take a bite. And unless you remove the colour from any vegetable (save perhaps a parsnip which is a similar colour to a potato), even getting the fry to look exact;y like the one made of potato is going to be very difficult - not to mention that the properties of most vegetables are different so if you fry or bake a parsnip or other vegetable like a French fry/chip, they often do not match in final texture (most will be 'soggier' than potato fries/chips). Perhaps a kid who has little life 'eating' experience will buy it, but, I doubt if many adults can be so fooled.
p.s. If you really absolutely have to do this, it will probably necessitate taking everything down to the chemical/molecular level and recreating foods with 'chemicals' (examples might exist in the flavourings added to various potato chips to 'simulate' salt and vinegar or barbeque or chicken teriyaki tastes on the surface - most of which fail abysmally and may not be exactly healthy anyway). TVP is another possibility - soy masquerading as x, y or z - but many of us should not be consuming that stuff either!
