Place cut side down on a baking tray. Bake for 40-50 minutes until soft.
Meanwhile, slice the bacon width-wise into strips. Add to a frying pan and sauté until it is crispy and brown. Then remove the bacon to a paper towel and drain.
Sauté the onion until translucent in a tablespoon of the bacon grease. Add garlic and sauté until soft.
Allow the squash to cool enough for you to handle. Once the squash is cool, shred the meat with two forks to form the 'spaghetti'.
Once you remove the squash, toast the walnuts in the oven on the same sheet pan for about 5 minutes. Then turn them over and toast for another 5 minutes.
Mix "spaghetti" and all the rest of the ingredients, except nuts, in a large bowl. Mix lightly so the squash meat doesn't clump up. Taste the filling. Season it to taste with salt and pepper.
Once mixed, place the squash mixture in one side of the squash skin.
Bake squash long enough to heat through, about 15 minutes.
Sprinkle with the toasted walnuts and serve hot.
Notes
Leftovers can be refrigerated for 3-4 days. Reheat in microwave or, covered, in 350°F oven for about 20 minutes, until hot.