Heat the olive oil in a large, heavy saucepan over medium heat. Add the leeks and cook, stirring frequently until golden and soft, about 5 minutes. Add the garlic and simmer for 1 more minute, then add the amaranth grains, stock, bay leaf, and tomato paste and bring to a boil.

Then reduce heat to a simmer, cover and cook for 30 minutes.

Remove the bay leaf from the pot, add 1 cup of the beans, and use a handheld immersion blender to puree within the pot until smooth.

Stir in the remaining beans, the herbs, and the salt. Warm gently just to heat through, if desired, thin the soup with additional heated stock. Season with additional salt and pepper to taste and serve in deep bowls drizzled with a thread of oil.