In a heavy-bottomed pot, melt 4 tablespoons butter over medium heat. Cook, swirling occasionally, until foamy and golden brown with a nutty aroma.
Add squash, garlic, herbs, red pepper flakes, and salt. Cook 6–8 minutes until squash is softened and garlic is fragrant. Stir in sugar or honey if needed.
Add remaining butter and flour, stirring constantly for 2–3 minutes until nutty.
Slowly whisk in broth, bring to a gentle boil, and simmer 3–5 minutes until slightly thickened.
Add tortellini, season with salt, and cook 3–4 minutes until pasta is al dente and squash is tender.
Turn off heat. Stir in cream, spinach, and parmesan until silky and smooth.
Adjust consistency with broth, milk, or cream if desired. Season to taste and serve hot with extra parmesan.
Optional topping: Fry sage leaves in butter until crispy, then sprinkle over pasta before serving.