For the pasta: Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, 8 to 10 minutes. Place the pasta in a large bowl.
Heat 2 tablespoons of the oil in a large nonstick skillet over medium heat. Add the shallots, carrots and 1/4 teaspoon of the salt. Cook until the carrots begin to soften, about 4 minutes. Add the orange juice, courgette and the remaining 1/4 teaspoon salt. Bring to a boil. Cook until the juice thickens and the courgette is tender, about 2 minutes. Add the courgette mixture to the pasta.
For the dressing: Whisk together the mascarpone cheese, lemon juice, lemon zest and salt in a small bowl until smooth.
Pour the dressing over the pasta. Add the tomatoes and sausage slices and toss until coated. Season with salt.