First, slice the ends off of the Roma tomatoes. To core them, simply run a knife from one end cut to the other straight down into the tomato, without cutting all the way through. You should only be cutting through the outside wall. Then use your knife to separate the seeds and the core from the flesh of the tomato. Reserve the cores for another use or discard them. Once cored, cut the tomato pieces into thin strips and then chop the strips into small pieces. Transfer the prepared tomato to a mixing bowl. 

Slice the jalapeño pepper in half, remove the seeds and ribs then slice the pepper halves into thin strips. Chop the strips very finely and add to the tomatoes. 

Finely dice half of 1 large red onion - save the rest of the onion for another purpose. Add the onion to the bowl with the tomatoes and pepper.

Finely mince cilantro leaves and add to the bowl.

Squeeze the juice of 1- 2 limes over the top, drizzle with 1-2 tablespoons olive oil, and sprinkle heavily with coarse salt and freshly cracked pepper then stir to combine. Allow the flavors to marinate and meld together for at least 20 minutes before serving, preferably a few hours.