Halve the avocados and remove the pits. Peel the avocados, then chop them into large chunks.
Transfer the avocados to a glass bowl, then pour the lime juice over top. Lightly stir to coat while mashing some of the pieces smooth (leaving most still chunky).
Add the diced tomatoes, chopped red onion, serrano pepper, cilantro, and olive oil. Lightly stir to combine.
Serve immediately, or cover (airtight) and chill the avocado salsa until ready to serve.
Notes
Serrano peppers too spicy? Try jalapeño peppers as a substitute. Jalapeños (2,500 to 8,000 Scoville heat units) are anywhere from 2 to 10 times milder than serrano peppers (10,000 to 23,000 SHU).