In a large pan, heat oil over medium-high heat. Add onion and cook until softened. Add garlic and celery and cook a few minutes more, until garlic is fragrant and celery is tender.
Add stock. Increase heat to high and bring to a boil. Add pasta. Boil until pasta is al dente. Reduce heat to low, add spinach, and simmer for a minute or two. Remove from heat.
Add lemon juice, red pepper flakes, and salt. Stir briefly. Sprinkle individual servings with bacon or ham, if using, and cheese.