1 pound spicy or sweet Italian sausage, casings removed
1 lemon, very thinly sliced
6 no-boil lasagna noodles, preferably Barilla
Melt butter in a saucepan over high heat. Stir in flour; cook for 2 minutes. Whisk in milk. Bring to a boil, stirring. Reduce heat. Simmer for 1 minute. Remove from heat. Whisk in 3/4 cup cheese, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Stir in chard.
Preheat oven to 350 degrees. Cook sausage in a skillet over high heat, breaking up pieces, until no longer pink, about 4 minutes.
Cover lemon slices with cold water by 3 inches in a saucepan. Bring to a boil. Reduce heat; simmer for 7 minutes. Transfer to a paper-towel-lined plate using a slotted spoon.
Spread 1/4 cup sauce in an 8-inch square nonreactive baking dish. Top with 2 noodles, half the sausage, and 1 cup sauce. Repeat. Top with a layer of lemons, 2 noodles, then remaining sauce and lemons. Bake, covered with parchment-lined foil, for 27 minutes.
Remove from oven. Heat broiler. Uncover lasagna; top with remaining 1/4 cup cheese. Broil until bubbling, 2 to 3 minutes.