Position a rack in the middle of the oven and preheat the oven to 400 degrees F.
Bring a large pot of salted water to a boil. Remove the stems from the chard but keep the leaves whole. Add the chard to the boiling water (the pot will be packed at first) and cook for 10 minutes. Drain in a colander and run under cool water. Drain again, then wrap the chard in a kitchen towel and squeeze to remove the excess water. Chop the chard.
Meanwhile, heat the butter in a saucepan over medium heat. Add the flour and cook, whisking, for 1 minute. Whisk in the milk and season with salt, pepper and a little nutmeg. Simmer until the sauce is thick enough to coat the back of a spoon. Adjust the seasonings to taste. Stir in 1/3 cup Roasted Garlic Paste.
Spread half the chard in the bottom of a medium casserole dish. Top with half the garlic sauce and half the cheese. Repeat the layers, ending with the cheese. Bake for 15 minutes. Remove the casserole from the oven and turn on the broiler.
Halve the bread lengthwise, then cut in half crosswise to make four 12-inch-long pieces. Broil the bread, cut-side up, until charred, about 1 minute. Slather with the remaining Roasted Garlic Paste and top each piece with one-quarter of the chard au gratin. Cut each pizza in half and serve.
The unbaked chard au gratin can covered be and refrigerated for a make-ahead meal. Bring to room temperature, then bake as directed.
Roasted Garlic Paste:
Preheat the oven to 425 degrees F. Arrange the garlic in a cluster on a large piece of foil and drizzle with 1/4 cup EVOO. Wrap up the garlic in the foil to make a tightly sealed pouch. Roast until very tender, about 45 minutes. Let cool, then squeeze the garlic from the skins into a food processor. Add the remaining 3/4 cup EVOO and the honey and season with salt and pepper. Puree until smooth, then transfer to a small container. (Alternatively, you can mash the garlic paste with a fork instead of using a food processor.)