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