Pre-heat the oven to 400°F with a rack in the middle of oven.
Bring a large pot of water to boil.
Stem the chard, leaving the leaves whole. Salt the boiling water, add the chard and wilt – the pot will be packed at first.
Boil the chard for 10 minutes, then drain it in a colander and run it under cool water. Let the chard drain once again and squeeze out the excess liquid in a clean kitchen towel. Chop.
Meanwhile, heat the butter over medium heat. Whisk in the flour, cook 1 minute, then whisk in the milk. Season with salt, pepper and a little nutmeg. Stir in the roasted garlic paste. Thicken the sauce to coat the back of a spoon and adjust the seasonings, to taste.
Layer half of the greens in a medium casserole (8-10” long). Top with half of the béchamel sauce and half of the cheese. Repeat the layers, ending with cheese. Bake for 15 minutes and serve.