Preheat oven 425. Halve squash lengthwise and scoop out seeds. Cut crosswise into 1/2” slices. On rimmed baking sheet toss in EEVO, season with salt/pepper. Roast 10min+ till lightly browned.
Heat butter in nonstick skillet and whisk in flour for 1 min. Whisk in milk till thickens, about 3-5min. Season w/ salt/pepper. Remove from heat and stir in 1/2 of Parmesan. Arrange squash, pushing into bechamel sauce, then sprinkle w/ remaining Parmesan. Bake 10-12min.