Preheat oven to 375 degrees. In a small pot, bring broth, milk, and garlic to a boil over medium-high and cook 1 minute. Pour into a medium bowl, then slowly add flour, whisking constantly. In pot, melt butter over medium. Add anchovies and 2 teaspoons sage and cook, stirring, 1 minute. Return broth mixture to pan, bring to a simmer, and cook until thickened, 6 minutes. Discard garlic.
Add squash to sauce and season with salt and pepper. Transfer to a 2-quart shallow baking dish. In a small bowl, combine 1 teaspoon sage, breadcrumbs, and Parmesan and sprinkle over squash. Bake until top is golden brown and mixture is bubbling at edges, 20 to 25 minutes.
Made this tonight to go with simple pan-broiled porkchops, and it was a hit with the adults at the table. The kids thought it looked unappetizing. That said, it was a delicious side that tasted much richer than it was. My grocery was out of fresh sage so I subbed 1 tsp dried rubbed sage, and I used plain dry breadcrumbs instead of fresh. I also mixed in a little tiny bit of melted butter since dry breadcrumbs don't work well by themselves. Used anchovy paste instead of anchovies... Delish!
I made a half portion of this recipe just last night and I have to say that it was excellent, a real crowd pleaser. It was easy, quick and tasted great. I did modify the topping slightly - sub'd panko for the bread crumbs, and added butter to the panko along with the Parmesan and fresh sage. Yum! I'll definitely make this again!