Thinly slice the green onions, keeping the white bottoms separate from the green tops. Transfer green tops to a small bowl; set aside for serving. Add white bottoms to a large skillet. Add ground beef and sweet pepper to skillet. Cook over medium heat until meat is no longer pink, using a potato masher or wooden spoon to coarsely break up meat as it cooks. Drain off fat if necessary.
Sprinkle meat mixture with cumin. Add salsa. Cook and stir over medium heat for 1 minute. Transfer meat mixture to a medium bowl; set aside.
While meat is cooking, place cauliflower florets in a food processor. Cover and pulse until finely chopped (cauliflower should resemble rice).
In the same skillet used to cook the meat mixture, heat olive oil over medium heat. Add finely chopped cauliflower; sprinkle with ½ teaspoon salt. Cook, uncovered, for 5 to 6 minutes or until cauliflower is just tender, stirring occasionally.
Add meat mixture to cauliflower rice. Cook and stir over medium heat for 1 to 2 minutes or until combined and heated through.
In a small bowl combine avocado, lime juice, garlic, and 1/8 teaspoon salt. Mash with a fork or potato masher until well combined and to desired consistency.
To serve, divide cauliflower mixture onto four serving plates. Top with avocado mixture, reserved green onion tops and, if desired, additional salsa and fresh cilantro.