Heat oil in a 5 to 6-qt. saucepan or Dutch oven over medium heat; add garlic and red pepper flakes and cook 30 to 45 seconds, just until garlic is lightly toasted. Remove pan from heat and slowly stir in broth.

Return pan to heat. Bring broth to a boil over high heat; add tortellini. Reduce heat to medium; cover and simmer 1 minute less than recommended on tortellini package.

Stir in spinach, basil, lemon juice and zest; cook about 1 minute or just until spinach is wilted. Serve immediately with additional basil and shredded Parmesan cheese, if desired.