Prepare vegetables. Rinse asparagus and trim the woody ends off. If you snap a piece of asparagus, it will typically break at the tough end. Discard the ends. Dice the remaining asparagus into pieces about 2 inches. Cut the green top off the leek and discard. Slice the leek crossways. Trim the top off the fennel bulb and slice the fennel lengthwise. Measure peas and set aside.
Heat oil in a heavy pot over medium heat. Add shallots, asparagus, leek, fennel and salt. Saute for about 4 minutes. Add navy beans, peas and vegetable broth. Bring to a boil and simmer for about 15 minutes.
Add fresh mint and dill. Simmer for another couple of minutes.
Let soup cool and transfer to a blender. Blend to velvety consistency. If you have a vented blender such as Vitamix, you do not have to wait for the soup to cool. Transfer back to pot. Add lemon juice and some black pepper. Add more vegetable broth if you prefer the soup less thick. Reheat and serve.
Optional: You can also saute some extra diced asparagus and peas and add them back to the soup for more texture. You can also top with some extra chopped fresh dill.