In a large pot over medium heat, melt butter. Add onion and cook until soft and translucent, about 4 to 6 minutes. Add garlic and cook until fragrant, about 1 minute more. Add Italian seasoning, red pepper flakes, and flour, whisk to combine, and cook 1 minute more.
Add tomato paste and cook until darkened slightly, 2 to 3 minutes. Add broth and tomatoes and bring to a boil. Lower to a simmer and cook until tortellini is tender and cooked through, about 10 minutes.
Add the cream and Parmesan and stir to combine, then add spinach and let wilt. Season to taste with salt and pepper, garnish with basil, and serve.