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But I know some people who would agree with you that it's a topping and not a sandwich. 
You should try it. It's delicious.
There may not be Duke's mayonnaise for sale where you live. That's a shame, and you should really try to get some, because it's markedly less sugary than other commercial mayonnaise brands and allows the tomato slices to sing their luscious, sweet and tangy tune.
How to win over a mayo-hater
Hellmann's will also get the job done, but if anyone begins to bring up the possibility of making the mayonnaise for this sandwich at home ("It's sooooo eeeeaaasssyyy. Just use your bllleeeenderrrr..."
, banish them to the porch until they have contemplated the error of their ways. Yes, even if it is raining. Simplicity is serious business here.
Same goes for the white bread. You must not make this bread, nor should the word "artisanal" be uttered within 100 paces of it. You must purchase this bread and the word "crappy" must be at least somewhat applicable to it. Chef Bill Smith of Crook's Corner restaurant in Chapel Hill, North Carolina makes his with store-bought bread (a move New Orleans chef Adolfo Garcia reportedly referred to as "ballsy" and the man's won or been nominated for every big cooking award under the sun. Trust him, for he is a professional maker of tomato mayo sandwiches.
Upon this soft, crappy bread, slather the mayonnaise. How thickly and on one or both slices - that's your business. On top of one slice, layer tomatoes.
The tomatoes should come from a farm, a farmstand, a neighbor or if you're extremely lucky, your own garden. If the angels are smiling upon you from the heavens and you saved a basket of kittens from certain death on a railroad track in a past life, these tomato will be of an heirloom variety. They should be red (yes the yellows, oranges and purples are stunning to behold, but we're on a particular mission here) and taste of blue skies and blazing sun. At the very least, they should have been grown in soil rather than a hydroponic compound, but sometimes, we must make do. If they have seen the inside of a fridge, though, skip them. These are not the tomatoes you're looking for.
Cut the slices to whatever depth brings you the greatest pleasure. For some, this will be akin to the thickness of a thumb. Others may wish to skim this month's copy of Nightshade Enthusiast through theirs. Either way, you're in it for the juice - or rather the locular jelly, which is that luscious goop in the center that holds all the acid. There should be enough of that to stain the mayonnaise a light pink and make your knees buckle just a little bit.
When you're finished layering the slices. Stack on the top slice and...wait. It'll taste good right now, but it'll be even better in ten or fifteen minutes when the juice has had a chance to seep in and meld with the mayonnaise and juuuuust begin to sog up that first millimeter or two of bread. You've held out all year for tomatoes to be in season - what's a few minutes more?
http://www.cnn.com/2014/07/28/living/tomato-mayo-sandwich-eatocracy/
gonna have to get me some today.
Accept no substitute for slathering onto both slices of your RAW toast (it ain't a BLT!):
Warm the sun. It just doesn't get any better than that. 
