Heat the olive oil in a large pot or dutch oven over medium heat. Add the onion and celery. Cook, stirring occasionally, until the onion and celery is soft, about 3–4 minutes.

Add the vegetable stock and water and increase heat to high. Bring the stock to a boil and add the sweet potatoes. Reduce heat to medium low. Cook for 10 minutes, or until the sweet potato is almost tender.

Stir in the beans, chopped spinach, thyme, lemon juice, and salt. Cook for 1 minute, or until the spinach is wilted. Remove from the heat and ladle into bowls. Serve with a spoonful of pistou.

Bring a small pot of water to a boil. Prepare a medium sized bowl with ice water.

Place the basil leaves in the boiling water. Let cook for 15 seconds. Use a slotted spoon to remove the basil and place the basil into the ice water. Let sit for 30 seconds in the ice water.

Use a slotted spoon to remove the basil from the ice water and place on a clean kitchen towel. Pat dry with another clean kitchen towel. Gently roll the basil up in the towel to dry it some more (don’t squeeze as the basil is easily bruised).

Place the garlic in the bowl of a food processor. Pulse until chopped. Add the olive oil, salt, and basil. Pulse gently, just until combined and the basil is chopped into tiny pieces (it doesn’t need to be completely smooth). Pour the pistou into a small bowl and serve a spoonful with each bowl of soup.