1 1/2 pounds (about 700grams) tomatoes- rougly 6-8 medium tomatoes
2 tsp salt
Finely dice the red onion and mix in a bowl with the lime juice. This step should be done first, so that the onion can macerate in the lime juice, which will help subdue the strong flavors of both ingredients.
Finely chop cilantro and dice tomatoes into small cubes. Add into bowl.
Seed and mince jalapeno, adding into bowl as well. Season with salt. Before you adjust the seasoning, store in fridge for at least an hour or two to let the flavors meld together- the acidity and spiciness of the onions will assuredly mellow, and the salsa will deepen in tomato flavor. Lots of liquid will release, so strain if you would like (I prefer to serve it as-is). Taste before serving and adjust seasoning then.
Salsa can be stored in the fridge for 3-4 days. That is, if you have the restraint to not use it on everything.