Heat a large, high-sided skillet over medium-high heat, then add 2 tablespoons of the butter. When it is melted, foamy, and beginning to brown, add the onion and cook until translucent and beginning to brown, 3 to 5 minutes. Add the garlic, sausage, and sun-dried tomatoes. Season with black pepper and 1/2 teaspoon salt and stir to combine. Cook, stirring occasionally, until the sausage begins to brown, 8 to 10 minutes. Transfer the sausage mixture to the pot with the pasta and stir to combine. Reduce the heat under the skillet to medium-low.