First thing is to hull and dice the strawberries. I like them cut rather tiny but if you are a size queen, you can make them bigger. It's just a visual thing. By the way, did you know that strawberries are the only fruit with their seeds on the outside?
Strawberries are pretty fragile as are avocados, especially when they are cut into such small cubes. Maybe fragile is the wrong word. Let's just say that you don't want to over mix this salsa as it will get mushy quickly. So first combine the 1/4 cup red onion, minced serrano pepper, the 1/4 cup cilantro, 1/2 lime juice and the sugar and salt, mixing well.
Now let's cut the avocado into cubes. Watch my video on How to Seed an Avocado. Then using a knife, cut a grid into each half. Then scoop out each side into the bowl with the other ingredients.
Add in the strawberries and fold the mixture.
Because of how quickly this salsa can go from being attractive to not, I suggest you not make it too far ahead of time. Also, serve it at room temperature. Strawberries are their most flavorful then.