Cook the tortellini according to package directions in a large pot of salted boiling water, then drain and set aside.
In a mixing bowl, whisk together the milk, heavy cream, and flour until smooth; set aside.
Heat a large skillet over medium heat and add the diced tomatoes with their juices, minced garlic, chopped spinach, salt, black pepper, onion powder, garlic salt, and Italian seasoning.
Bring the tomato mixture just to a simmer (small bubbles around the edges).
Slowly pour in the milk–cream–flour mixture while stirring constantly to combine, then add the grated Parmesan and continue to cook until the sauce comes back to a gentle boil.
Reduce the heat and simmer 2–3 minutes, stirring occasionally, until the sauce thickens slightly.
Add the cooked tortellini and shredded mozzarella to the sauce and stir until the pasta is fully coated and the cheese is melted.
Serve warm.