Arrange the rack in the middle of oven and preheat the oven to 400 degrees F. Bring a large pot of water to boil.
Stem the chard, keeping the leaves whole. Salt the boiling water and add the chard and let wilt. The pot will be packed at first. Boil the chard 10 minutes, drain in a colander and run under cool water. Let it drain and squeeze out any excess liquid in a clean kitchen towel. Chop.
Meanwhile, heat the butter over medium heat. Whisk in the flour and cook 1 minute, and then whisk in the milk. Season with salt, pepper and a little freshly grated nutmeg. Stir in the roasted garlic paste. Thicken the sauce to coat the back of a spoon and adjust seasonings to taste.
Layer half the greens in a medium casserole (8 to 10 inches long). Top with half the bechamel sauce and half the cheese. Repeat layers, ending with the cheese. Bake until bubbly and brown, 20 to 30.