Cut the Roma tomatoes into evenly sized ¼-inch cubes. Finely chop the onion to a similar size so it blends consistently throughout the salsa.
Remove the jalapeño seeds for a gentler flavor, or retain some or all of them for greater heat. Mince the pepper finely, then roughly chop the cilantro leaves.
Place the tomatoes, onion, jalapeño, and cilantro in a medium mixing bowl. Sprinkle in the salt and black pepper, then stir carefully until the seasonings are evenly distributed.
Allow the salt to begin drawing a small amount of liquid from the tomatoes, forming a lightly seasoned juice that flavors the entire mixture.
Pour the freshly squeezed lime juice over the seasoned vegetables and fold it in gently.
Halve the avocado, remove the pit, and scoop the flesh from the skin. Cut or break it into bite-sized chunks directly over the bowl.
Using a spatula, carefully fold the avocado into the tomato mixture without crushing the pieces. Taste and adjust the salt or pepper as necessary.
Serve immediately to preserve the avocado’s fresh color and firm, creamy texture.