Bring a large pot of salted water to a boil and cook the rotini according to package directions until al dente; reserve about 1/2 cup of the pasta cooking water, then drain.
Meanwhile, heat the butter and olive oil in a large skillet over medium heat until the butter melts.
Add the chopped onion and cook 3 to 5 minutes until softened and translucent.
Add the minced garlic and cook about 30 seconds until fragrant.
Stir in the tomato passata, Italian seasoning, crumbled bouillon cube, sugar, red pepper flakes, salt, and pepper; simmer for about 5 minutes to meld flavors.
Pour in the half and half, stir, and simmer 2 to 3 minutes until the sauce is slightly reduced and creamy.
Add the drained pasta and shredded mozzarella to the skillet and toss to combine, cooking a few minutes until the cheese melts and the pasta is heated through; add reserved pasta water a splash at a time if the sauce is too thick.
Serve immediately, optionally garnished with fresh parsley or basil and grated Parmesan if desired.
