Heat oil in a large stockpot over medium-high heat. Add onion, fennel, carrot and celery. Sauté for 6 minutes, stirring occasionally.
Add garlic and crushed red pepper flakes. Sauté for 2 minutes, stirring frequently.
Add the chicken stock and stir to combine. Continue cooking until the soup reaches a simmer.
Add the tortellini, spinach, lemon zest and juice, fresh dill. Cook, stirring occasionally, until the tortellini are cooked through.
Taste and season the soup with salt, pepper, and extra dill or lemon juice if needed.
Serve warm, garnished with lots of freshly-grated Parmesan cheese.
