Add 15 ml of olive oil to a 4-5 quart sauté pan over medium-high heat. Allow the oil to heat for a minute until it shimmers.
Add 450 g of sliced smoked kielbasa (or turkey/chicken sausage) and 225 g of diced onion to the pan. Sauté for about 5-7 minutes until the kielbasa is browned and the onions are soft.
Once cooked, add 2 cloves of minced garlic to the pan. Cook for an additional 30 seconds until fragrant.
Pour in 500 ml of low-sodium chicken broth and 280 g of diced tomatoes (including their juice). Stir to combine all ingredients.
Add 225 g of dry small pasta, 120 ml of milk or heavy cream, 2.5 g of salt, and 2.5 g of pepper. Stir to combine, ensuring the pasta is submerged in the liquid.
Bring the mixture to a gentle simmer. Cover and cook for about 15 minutes, or until the pasta is tender. Stir occasionally.
Remove from heat and stir in 100 g of shredded Cheddar cheese until melted.
Top with remaining cheese and cover until melted, spotty brown, and bubbly.
Sprinkle with 30 g of thinly sliced scallions and serve immediately.