Preheat oven to 350°F (180°C). Drizzle fennel and leek with olive oil, season lightly with salt and herbs, and roast for about 20 minutes until soft and lightly golden.
Heat olive oil in a large pot. Sauté the onions, garlic, and green part of the leek until fragrant and translucent.
Add the diced potatoes and remaining herbs, stirring for a few minutes.
Pour in the vegetable broth and simmer until the potatoes are tender.
Add the roasted fennel and leek, green peas, and spinach. Blend until smooth and creamy.
Stir in nutritional yeast, adjust seasoning with salt and black pepper, and finish with lemon zest.
Serve warm with fresh herbs, croutons, or your favorite toppings.