Begin by adding the olive oil to a large dutch oven or heavy-bottomed pot. Warm it over medium-high heat. Once hot, toss in the diced onion. Sauté for about 5 to 7 minutes, stirring occasionally until the edges begin to brown. At this point, reduce the heat to medium.
Now, add the minced garlic and a pinch of black pepper. Sauté for about 1 minute, allowing the mixture to become fragrant. Next, add the zucchini and potato to the pot. Give everything a good mix, ensuring the veggies are well-coated in the oil. Let them sear on the bottom of the pot for about 3 minutes until they start to brown.
Here’s where the magic happens! Pour the vegetable broth into the pot, scraping any browned bits off the bottom with a spatula. Then, add the water. Bring the mixture to a boil over high heat. Once boiling, reduce the heat to medium and let it simmer for 15 to 18 minutes. Stir occasionally. The soup is done when the zucchini turns translucent and the potatoes can be easily pierced with a fork.
Carefully transfer the hot soup to a blender (be cautious of steam!). If your blender is smaller, you may want to do this in batches. Add the fresh basil and lemon juice to the blender as well. Blend on high for 45 to 60 seconds until smooth.
Return the blended soup to the pot. If needed, warm it on low heat. Season with salt and pepper to taste. Serve it warm, topped with toasted croutons, a sprinkle of cheese if desired, and a drizzle of olive oil. If you have any leftovers, store them in an airtight container in the refrigerator for up to 5 days or freeze for up to 2 months.
