Preheat oven to 375 degrees. Using a large, sharp knife, carefully slice spaghetti squash in half. Place face-up on baking sheet, drizzle with olive oil and sprinkle with salt and pepper. Bake at 375 for 55 minutes for two small squashes or up to 75 minutes for a large. Strands should be only slightly crunchy when they're done cooking.
While squash is in the oven, sauté bell pepper and red onion over medium heat until softened but still slightly crunchy. Add enchilada sauce, black beans, chicken, fresh cilantro, and salt and pepper to taste. Stir mixture and heat through. Keep on low heat.
When squash is done cooking, remove baking sheet from oven, scoop out seeds and gooey innards, and drain any water that may have collected inside the squash during cooking. Fork through the squash to loosen the strands from the shell.
Turn oven up to low broil setting. Fill spaghetti squash shells with pepper mixture and stir to mix the squash strands with the filling. Top each squash half with shredded cheese and return the baking sheet to oven for 1-2 minutes, until cheese is just melted under the broiler. Serve with guacamole, plain Greek yogurt or sour cream, and salsa.
To prep in advance, cook the spaghetti squash fully (and the filling if desired) ahead of time and refrigerate. Then, increase the baking time to 40 minutes and bake at 350 degrees, covering with foil for the first 20-30 minutes.The best spaghetti squashes are the ones that are bright yellow! The pale ones tend to be more watery.