Remove casings from sausage, if necessary, and place in a medium skillet over medium-high heat. Break up sausage with a spatula and cook until browned and cooked through, but still, has some color. Remove from heat and set aside. Add onions, pinch of salt and 1-tablespoon butter to skillet. Cook until onions begin to soften and caramelize. Mix in kale with a pinch of salt and pepper and cover skillet until leaves begin to wilt. Remove from heat, add lemon juice, mix and set aside. Pile half of Wisconsin kasseri on 1 slice of Pane Turano and layer with kale mixture, sausage and remaining cheese, in that order. Place the second piece of bread on top and cover outsides of both bread slices with remaining butter. Grill sandwich in a pan over medium heat, turning once until both sides are golden brown and cheese has melted. Let sandwich sit for 1 minute before serving – this will help the cheese thicken which helps the sandwich stay together as you're eating it.