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. Remove from heat and set aside.
Add onions, a pinch of salt, and 2 tablespoons 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.
Divide half of the provolone between four slices of lightly toasted bread and top with kale and onion mixture, browned sausage, and remaining cheese.
Melt remaining butter in a pan over medium heat. Toast open-faced sandwiches, in batches if necessary, until cheese is melted and bread is golden brown. Cover pan with lid to allow steam to melt the cheese.
Let melts sit for one minute before serving.
Recipe adapted from: www.thespruceeats.com