Preheat the broiler and position a rack 8 inches from the heat. Set a steamer basket in a large, deep skillet filled with 1 inch of water and bring to a boil. Add the broccoli spears, cover and cook over high heat until crisp-tender, about 5 minutes. Transfer the broccoli spears to a shallow 1 1/2 quart baking dish, arrange them in a single layer and season with salt.
Meanwhile, in a medium saucepan, melt the butter. Add the flour and whisk over moderately high heat for 2 minutes. Add the half-and-half and cook, whisking constantly, until thickened and bubbling, about 5 minutes. Remove from the heat and whisk in the cheddar and chipotle.
Pour the cheese sauce over the broccoli. Broil for 5 minutes, shifting the dish once halfway through, until the gratin is bubbling and browned in spots.