I have some leftover microwaved potatoes. Can I use these potatoes in Spanish omelette? Also, I read that tomatoes are not part of "authentic" Spanish omelette. Is that true?
You shouldn't use boiled potatoes as you have to fry them.
This is a good recipe, and no, tomatoes are not part of "authentic" Spanish omelette. In Spain we call it "Potato omelette" an the only ingredients are eggs, potatoes, olive oil, salt and sometimes onion.
Being grown in Spain I've eaten boiled potatoes Spanish omelette a few times: when my mother wasn't in aim for cleaning the splattered oil after cooking. She stopped making omelettes that way: it simply didn't have the taste it should have.
IMHO potatoes should be deep fried in olive oil as that will give them a crust/scratching outer part with an inside part with the consistency of boiled potatoes. To archieve this I heat oil up to 180C (350F) and when I put the potatoes in, immediatelly set the temperature at 120~140C (250~280F), having the potatoes an irregular cutting.
I've seen potatoes omelette having added: onion (wheels or slices), green pepper (sliced, never red, don't know why), cooking chorizo or even jamon serrano dies (between .5 and 1cm (.93in.) cubes). But never seen tomato being added to a potatoes omelette.