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 canned diced tomatoes with their sauce, minced garlic, chopped spinach, salt, black pepper, onion powder, garlic salt, and Italian seasoning.
Bring the tomato mixture to a gentle boil, stirring occasionally.
Whisk the reserved milk-and-flour mixture again, then pour it into the skillet and stir to combine.
Add the grated Parmesan cheese and cook, stirring, until the sauce comes back to a simmer and begins to thicken.
Reduce heat and simmer for 2–3 minutes to let the sauce thicken slightly.
Toss the drained tortellini into the sauce and stir until fully coated, then sprinkle in the shredded mozzarella and stir until melted and combined.
Serve warm.
