1. Add olive oil to a 4-5 quart sauté pan over medium-high heat.
2. Fry the smoked kielbasa and onions until the onions are soft and the kielbasa is browned.
3. Add the minced garlic and cook until fragrant, about 30 seconds.
4. Add chicken broth, diced tomatoes (with juice), heavy cream, pasta, salt, and black pepper. Stir to combine.
5. Bring the mixture to a boil, then reduce the heat to low and simmer for about 15 minutes, or until the pasta is tender and the sauce has thickened.
6. Remove the skillet from the heat and stir in ½ cup of the shredded Cheddar cheese until melted and creamy.
7. Sprinkle the remaining ½ cup of cheese on top. Cover the skillet until the cheese is melted, spotty brown, and bubbly.
8. Sprinkle with thinly sliced scallions before serving.