Heat the olive oil in the skillet over medium heat. Add the chard stems and a pinch of salt. Cook, stirring frequently, until tender, about 5 minutes. Fold in the chard leaves, and cook another 2 minutes, until wilted. Add the butter, lemon zest and juice and stir until it forms a velvety sauce. Reduce the heat to low, then fold in the chopped tomatoes. Transfer the cooked spaghetti and the sausage to the skillet and toss to combine. Cook 2-3 minutes until warmed, then transfer to a serving bowl. Sprinkle with freshly cracked black pepper and Parmesan cheese and serve.