Wash the avocados, cut in half, and remove seed, and peel. Coarsely chop the avocados and place into a bowl. Add lime juice, stir the avocados and lime juice together, coat well, as this will help to prevent browning. Add in salt, jalapenos, cilantro, garlic powder, and gently mash the avocados breaking the coarsely chopped chunks into smaller pieces. You will want to completely mash about one-half of the mixture before adding the red onions and tomatoes. Stir to combine well. Cover bowl with plastic wrap, and refrigerate for at least 30 minutes before serving.
You can learn more about selecting the proper avocados by going to the California Avocado Commission.