Make the crespelle batter: In a medium bowl, whisk together the milk, eggs, and salt until well combined. Sift in the flour a little at a time, whisking after each addition, until the batter smooth. Whisk in the melted butter, then cover and refrigerate the batter for at least 30 minutes or up to 24 hours.
Meanwhile, make the béchamel sauce: In a large pot over medium heat, add the butter, and melt completely. Whisk in the flour, and let cook, stirring continuously, until the roux is golden, frothy and thickened, about 3 minutes. Slowly whisk in the milk until incorporated, then turn the heat to high and cook, whisking continuously, until the milk begins to simmer. Lower the heat to medium-low and cook, whisking regularly and scraping the bottom of the pot to make sure the béchamel sauce doesn’t stick or scorch. Continue cooking until the sauce has thickened, 10–12 minutes. Stir in 2 teaspoons of the salt and the nutmeg and stir to combine. Cover the sauce with a sheet of plastic wrap pressed directly against the surface to prevent a skin from forming and set aside at room temperature (or transfer to the fridge for up to 48 hours).
Make the crespelle: Give the batter a quick stir in case the solids have settled at the bottom of the bowl. To a small nonstick skillet over medium heat, add a few drops of olive oil, then wipe the skillet clean with a dry paper towel so that only a very thin film of oil remains. When the skillet is hot, hold the skillet in one hand and ¼ cup ladle of batter in the other. Gingerly pour the batter into the skillet and immediately tilt the pan in a circular motion to spread the batter in an even layer across the surface. Cook the crepe until the edges become golden brown and the underside has developed a golden color, 2–3 minutes. Using your fingers or a thin spatula, flip the crepe and cook until the batter is fully set and begins to turn golden in color, about 30 seconds more. Transfer the crespella to a large plater. Repeat with the remaining batter, adding more oil to the skillet as necessary if it becomes too dry. You should have 10 crespelle by the end of this process. If preparing the crespelle in advance, you can stack them on a plate, wrap them in plastic wrap, and store them in the refrigerator for up to 24 hours.
Preheat the oven to 450°F.
Make the filling: Stack the chard leaves atop one another, roll them into a tight log, then slice crosswise, creating ¼-inch ribbons; set aside. Heat a large skillet or Dutch oven over high heat and add the canola oil. Using a cutting board or a plate, divide the sausage meat in half and press each half into a thin layer (the more surface area you create, the more tasty caramelization you’ll get). When the oil is hot and just beginning to smoke, add half of the sausage to the skillet and cook, undisturbed, until it turns golden on one side, 2–4 minutes. Use a wooden spoon to break up the meat into small pieces, stirring and scraping the bottom of the skillet. Continue cooking until the meat is cooked through, 1–2 minutes more, then transfer the cooked sausage to a large bowl. Repeat with the second half of the sausage. Once the second half of the sausage is cooked, add the first half back to the skillet, along with the onion and black pepper. Lower the heat to medium and cook, stirring regularly, until the onion has softened, 6–8 minutes. Add half of the chard along with 1 teaspoon of salt and cook until the chard is wilted but still bright green, about 2 minutes. Add the remaining chard and salt and cook until all of the chard is fully wilted, 3–4 minutes more. Remove from heat, then stir in 2 cups of the reserved béchamel sauce and set aside until cool enough to handle.
Place one of the crespella on a large, clean plate and scoop ½ cup of the sausage-chard mixture into the center, using a spoon to spread it into an even line Roll the crespella into a tube shape and transfer the roll, seam-side down, to a 9- by 13-inch baking dish. Repeat with the remaining crespelle and filling. Spoon the remaining béchamel sauce evenly over the top of the crespelle, sprinkle with the grated Parmigiano-Reggiano, and bake until the top is golden brown and the edges are crispy, 15–20 minutes. Serve hot.