Heat oven to 400°F 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.