Prepare the Beans:
Heat olive oil in a medium saucepan over low to medium heat. Add onion, garlic, carrot and celery. Sauté for 5 minutes. Add 1/4 cup of broth and sauté for 5 more minutes. Add remaining 1/4 cup of broth, beans, sage, thyme and salt. Cover, reduce heat to low and continue cooking for 20 minutes, letting the beans soak up the flavors. Remove from heat, remove the herbs and blend in a blender or mini food processor until smooth and creamy. Add a little chicken broth if needed. Taste and adjust for salt.

Prepare the Prawns:
Put 2 quarts of water in a large saucepan and bring to a boil, over high heat. Add fresh prawns and salt. Remove from heat, cover and let sit for 5 to 7 minutes. Drain.

Prepare the Dressing:
In a small bowl, whisk the olive oil with the lemon juice, salt and pepper until thoroughly mixed. Combine the cooked and drained prawns with the dressing. Let marinate at room temperature for at least 2 hours.