Preheat the oven to 200°C.
Finely slice the onions and cook them over low heat in butter until soft. Add sugar, salt, and pepper, then set aside.
Remove the Nduja sausage from its casing and brown it in a pan.
Once well browned, add the tomato coulis, oregano, and basil. Set aside.
Mix the cold béchamel sauce with the shredded mozzarella.
Spread a layer of béchamel and mozzarella at the bottom of a large baking dish. Cover with the tomato sauce, place lasagna sheets on top, then add the caramelized onions. Repeat the process until all ingredients are used.
Finish with a layer of béchamel and mozzarella, then bake for 25 minutes.