Dice up equal quantities of onion and tomato. Roughly chop the cilantro.
Now, slice 1 or 2 jalapenos in half. With a spoon, scrape out the seeds. (If you like things spicy, leave in some of the white membranes.) Dice the jalapenos very finely; you want a hint of heat and jalapeno flavor, but you don't want to cause any fires. Now dump the four ingredients into a bowl.
Slice the lime in half and squeeze the juice from half a lime the bowl. Sprinkle with salt, and stir together until combined. Be sure to taste the pico de gallo and adjust the seasonings, adding salt or more diced jalapeno if needed.