Cook the tortellini according to package directions in a large pot of salted boiling water; 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 sauce, minced garlic, and chopped spinach; stir in salt, pepper, onion powder, garlic salt, and Italian seasoning.
Bring the tomato mixture just to a boil, then immediately pour in the milk, cream, and flour mixture while stirring to combine.
Add the grated Parmesan and stir until the sauce comes back to a gentle boil and begins to thicken.
Reduce the heat and simmer for 2–3 minutes, stirring occasionally, until the sauce thickens to your liking.
Add the cooked tortellini to the skillet and toss gently to coat evenly with the creamy tomato sauce; sprinkle in the shredded mozzarella and stir until melted and combined.
Serve warm.