Cook Pasta:

Bring a large pot of salted water to the boil then cook pasta according to package directions. Once cooked, reserve ½ cup of hot pasta water then drain pasta.

Prepare Avocado Sauce: 

Cut tomato in half then scoop out any loose seeds and discard. Then, chop into small pieces and set aside.
Add avocado flesh to a large bowl then use a fork to mash into a chunky paste. Add the garlic, lemon juice, salt and a dash of pepper. Mix. Taste then adjust salt and or pepper as needed.

Combine Sauce and Pasta:
Add the drained cooked pasta, about ¼ cup of the reserved pasta water and the tomatoes to bowl. Use two spoons or tongs to toss. The avocado will turn into a thin sauce that coats the pasta. Add more reserved pasta sauce if to thin the sauce out even more if needed.
Serve with green onions sprinkled on top.