Preheat oven to 400 degrees Fahrenheit. Slice the spaghetti squash in half lengthwise and scoop out the seeds. Season the cut sides of the squash with olive oil, salt, and black pepper. Place the squash halves face down on a baking sheet and bake for 35 to 40 minutes, or until tender. While the squash bakes, heat olive oil in a large skillet. Add chopped onion, bell pepper, and black beans, and cook until softened. If using, brown ground beef or turkey with the vegetables. Once the spaghetti squash is cooked, use a fork to pull out the 'strings' from the flesh. Stuff the squash halves with the prepared skillet filling. Top with shredded cheese, shredded lettuce, and sliced avocado before serving.
