Preheat your oven to 400°F (200°C). Carefully cut the spaghetti squash lengthwise from stem to tail using a large knife.

Lightly coat the flesh with cooking spray and place the squash cut-side down on a baking sheet. Bake for 30 to 40 minutes until tender when pierced with a fork. Once done, let it sit until cool enough to handle.

While waiting for the squash, heat the olive oil in a large skillet over medium heat. Add chopped shallots and minced garlic; sauté for about 3 minutes until soft and fragrant.

Sprinkle in the flour and cook for an additional 2 minutes, stirring frequently. Gradually pour in the reduced-fat milk, whisking continuously to prevent lumps. Allow the mixture to simmer gently, thickening for about 3 to 4 minutes.

Whisk in the lemon juice, grated Parmesan cheese, kosher salt, and freshly ground black pepper, then remove from heat and stir in the shredded rotisserie chicken.

Using a fork, scrape out and discard the seeds of the spaghetti squash. Carefully scrape out the spaghetti-like strands and stir them into the chicken Alfredo mixture until well combined.

Carefully fill the empty squash boats evenly with the chicken Alfredo mixture, then sprinkle mozzarella cheese over the top.

Return to the oven and bake for another 10 minutes. Set your broiler to HIGH and broil for an additional 2 minutes, or until the cheese is bubbling and golden in spots. Remove from the oven and garnish with freshly chopped parsley before serving.