Place spaghetti squash cut-side down in a microwave-safe dish. Pour in 1/4 cup water. Microwave on high until easily pierced with a fork, about 7 minutes.
Scrape insides of squash into spaghetti strands with a fork. Transfer to a bowl; add butter, salt, and pepper.
Heat olive oil in a large skillet over medium heat. Add garlic; cook and stir until fragrant, about 1 minute. Add to bowl with spaghetti squash. Add walnuts and a pinch of salt to the skillet; cook and stir until toasted, about 1 minute. Transfer to a small bowl.
Place sausage in the skillet; cook and stir until browned, about 5 minutes. Pour in Alfredo sauce and stir until heated through, about 5 minutes. Spoon sausage mixture over squash and top with walnuts.