Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente. Before draining, reserve about 1/2 cup of the starchy pasta water. This will help emulsify the avocado sauce later. Drain the pasta and set aside.

While the pasta is cooking, scoop the avocado flesh into a large mixing bowl. Mash it until smooth and creamy. Stir in the grated garlic, lemon juice, salt, and pepper. The lemon not only adds brightness but also helps prevent the avocado from browning too quickly.

Add about 1/4 cup of the reserved hot pasta water to the avocado mixture. Stir until the sauce reaches a silky, pourable consistency. If it seems too thick, add more pasta water a tablespoon at a time until smooth.

Add the drained pasta, diced tomato, and sliced green onion to the bowl. Toss everything together until the pasta is evenly coated in the avocado sauce. Taste and adjust with more salt, pepper, or lemon juice as needed. Serve immediately.