Add the celery, potatoes, carrots, onion, garlic, olive oil, thyme, salt, and pepper to a large pot. Pour in enough water to just cover the vegetables.
Bring to a boil, then reduce the heat and simmer for about 20–25 minutes, or until all the vegetables are very tender.
Meanwhile, prepare the chickpeas. Pat them thoroughly dry, toss with the olive oil, salt, and pepper, and spread them in a single layer on a baking sheet. Roast at 200°C / 400°F until golden and crisp, tossing once during cooking.
Once the vegetables are tender, reserve some of the cooking liquid. Blend the vegetables until completely smooth, adding enough of the reserved liquid to reach your preferred consistency. Taste and adjust the seasoning.
Ladle the soup into bowls and finish with a squeeze of lemon juice, chili flakes, and the crispy chickpeas.
Add fresh herbs, grated Parmesan or nutritional yeast, or garlic croutons, if desired.