Add the diced tomatoes, red onion, cilantro, lime zest and juice, jalapeno or serrano, garlic, cumin, salt, and black pepper to a blender or food processor, or use a tall-sided container if working with a stick blender. The tall container helps contain splatter, and the rough chopping gives the machine a cleaner, more even start.

Pulse in short bursts until the salsa is mixed and as chunky or fine as you like, stopping often to check the texture. Look for a mixture where you can still see distinct small pieces of onion and cilantro, not a uniform pink liquid.

Serve right away, or transfer the salsa to an airtight container and chill before serving if you want the flavor to settle a bit more. After a short rest, the sharp onion smell softens and the salsa tastes more balanced and blended.