Cut open your avocado to make sure it’s good. If it is, great! Set it aside. If it’s not, I’m sorry, you’ll need to use a different avocado.
Combine chopped red onion, lemon juice, and pinch of salt in a bowl to quick-pickle. Stir and let it sit while you prep the rest of the ingredients.
Mince the jalapeño and any other optional additions (cilantro, garlic, tomatoes, etc). Set aside on your cutting board.
Use a fork to scoop the avocado out of the skin and into the bowl with the lemon and onions. Use the fork tines to mash the avocado into large chunks.
Add the jalapeño, another few pinches of salt (~1/4 tsp), the ground black pepper, and any of the optional additions, to the avocado. Continue mashing with the fork or a muddler until you reach your desired consistency.
Adjust to taste — add more lemon, salt, pepper, etc if you think it needs it. Serve!