In a food processor or blender, mince the onion, garlic, and jalapeño with the lid on using the pulse button option until finely chopped. I usually give it two or three 4-second pulses. Open the food processor and push down any pieces that are stuck to the walls of the bowl.
Add the quartered tomatoes, then the cilantro. When you add the cilantro, it’s best to chop it first so that it all blends well with tomato. If it’s too crowded, some of the cilantro will get stuck at the top and you’ll have to open it, untangle it, and blend again.
Add the salt, lemon juice, and olive oil, and pulse another 2 or 3 seconds just to mix them into the salsa well. Remove the blade attachment, stir it all with a spoon, and pour it into a serving bowl.
Too much blending/chopping can make it turn into a tomato sauce, so pulse in short intervals to keep an eye on it. If you like a spicier salsa, add some of the seeds from the jalapeño.