In a saucepan over medium-high heat, bring the milk, broth and garlic to a boil and cook for 1 minutes. Turn off the heat and slowly whisk in the flour. Pour into a bowl.
In the same pot, melt the butter over medium heat and add the anchovies and 2 teaspoons of the sage. Heat for about a minute. Then pour the broth mixture back into the pot. Bring to a simmer and simmer for about 6 minutes or until slightly thickened. Scoop out the garlic.
Mix the cooked squash (see note) and the sauce either in your saucepan or right in your casserole dish. Season with salt and pepper.
In a small bowl, combine 1 teaspoon of the sage, the breadcrumbs and the parmesan cheese. Sprinkle this over the top of the gratin.
Bake until bubbly and golden, about 20-25 minutes.
Notes
To cook the spaghetti squash - prick the squash all over with a sharp knife. Bake on a baking sheet at 375°F for an hour and 20 minutes. Flipping it over about half way through. Remove from the oven and when it's cool enough to handle, cut it in half lengthwise and using a fork, scoop out the strands. Substitutions for sage - rosemary, thyme, Italian seasoning. Can add ½ cup of grated cheese when you mix the squash with the sauce.