Cook the tortellini: Bring a large pot of salted water to a boil and cook the tortellini according to the package directions. Before draining, reserve ½ cup of the pasta water.
Brown the sausage: While the tortellini cooks, heat a large skillet over medium-high heat. Add the Italian sausage and cook for 3–5 minutes, breaking it apart with a wooden spoon, until browned and cooked through.
Add the tomatoes: Stir in the diced tomatoes and cook for 1–2 minutes, just until they begin to soften.
Make the sauce: Reduce the heat to medium-low. Stir in the chicken stock concentrate, Italian seasoning, lemon juice, and ¼ cup of the reserved pasta water. Cook for 1–2 minutes, stirring until well combined.
Make it creamy: Reduce the heat to low and stir in the sour cream or Greek yogurt. Do not let the sauce boil. Add a splash more pasta water as needed until the sauce reaches your desired consistency.
Add the tortellini: Add the cooked and drained tortellini to the skillet and gently toss until evenly coated in the sauce.
Taste and serve: Season with more salt and black pepper if needed. Garnish with fresh parsley and serve immediately.