Preheat the oven to 400°F.

Line a baking sheet with parchment paper. Cut the ends from each spaghetti squash. Slice each squash in half lengthwise, and spoon out seeds from each half. Arrange the halves, cut side down, on the prepared baking sheet.

Roast the squash on middle rack of oven until it is fork tender, 50-55 minutes. While squash bakes, prepare the chicken Alfredo filling.

Set a medium saucepan over medium-high heat and add the butter. Once melted, add the minced garlic and saute until fragrant, about 30 seconds. Sprinkle the flour into the pan and whisk to form a paste. Cook for 30 seconds to 1 minute, then slowly pour the half-and-half into the pan, whisking to dissolve flour.

Cook the sauce, whisking constantly, until it begins to bubble and thicken. Once thickened, turn off the heat. Add the salt, black pepper, and Parmesan. Stir until cheese is melted. Fold in the chopped cooked chicken.

Remove baking sheet from oven and flip squash halves so they are cut side up. Use a fork to toss the strands in each half. Divide the chicken Alfredo mixture evenly among squash halves, gently packing it into the cavity. Sprinkle the shredded cheese evenly over each mound of filling.

Return baking sheet to middle rack of oven. Bake until cheese is melted and lightly golden and filling is gently bubbling, 10-15 minutes.

Check to see that squash is done. Remove from oven or add time as needed.

Transfer squash halves to serving plates and sprinkle with minced parsley, if desired. Serve immediately.