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The best tomatoes for pico de gallo are plum tomatoes. They have great flavor, a meaty texture, and contain less liquid than other types of tomatoes.  Look for plum tomatoes that are bright red and firm to the touch. Do not use overly ripe tomatoes as they contain a lot of liquid and will water down the pico de gallo. If plum tomatoes aren't available at your grocery store, dry-farmed tomatoes are another good option.  Use a sharp knife to dice the tomatoes into small cubes. Use a sharp knife to cut the onion into very fine pieces.  Most Mexican cooks use white onions in salsas, as white onions have a sharper taste than red or yellow onions. The sharp taste balances out nicely with the tomatoes in the pico de gallo.  If you can only find red or green onions, feel free to use them instead. Place the diced onions in a bowl and add the juice of half the lime to them. The acid in the lime will tame some of the sharpness in the onions. Use a sharp knife to finely chop up the garlic clove. Add the garlic to the bowl with the onions.  You can also use a garlic press, if available. Not all pico de gallo recipes use garlic, so if you don't like the taste of raw garlic, feel free to leave this ingredient out. Remove the leaves from the stems and chop them into fine pieces. Cilantro is not everyone's favorite herb. But it is a key element of a good pico de gallo, so be sure to add at least a little cilantro to the dish. If cilantro is not your thing, start with half the amount of cilantro called for and substitute the rest with parsley. It's important to remove the seeds from the peppers as they can be very hot.  Depending on the spiciness of the pepper, you can add as little as two peppers or as many as four peppers. Keep in mind Serrano peppers are spicier than Jalapeno peppers.
Dice the tomatoes and place them in a large bowl. Dice the onion. Mince the garlic. Cut up the cilantro. Seed and mince the peppers.