Cook bacon and cut up. Reserve bacon fat from pan. Add garlic and cook for a minute or two. Add flour to the pan and whisk for a minute. Add milk and mix until smooth. Add salt, pepper, lemon juice and basil and bring to a simmer.
Drain tortellini and add back to the pot. Turn to low heat and add spinach and most of the mozzarella, Parmesan cheeses and bacon. Add sauce and stir until well combined.
Add to an 8×8 or 9×9 dish. Top with the remaining mozzarella cheese, Parmesan cheese and bacon.
Cover your dish with foil and bake for 20 minutes. Remove foil and bake for an additional 5 minutes.