HEAT OIL in 4-quart pot. When hot, add celery, onion, garlic and sage. Cook until onion is softened, about 5 minutes. Add stock or broth and water. Bring to a boil. Add lamb and beans. Bring to boil again. Simmer, covered, 2 hours, stirring occasionally. Use slotted spoon to remove lamb. When cool enough to handle, remove meat from bone. Cut meat into 1/3-inch dice. Puree 2 cups beans with 1 cup soup liquid until smooth. Return meat and pureed mixture to pot. Stir well to combine. Add lemon juice and season to taste with salt and pepper. Can be made ahead and refrigerated as long as 3 days or frozen. Reheat gently, thinning with water as needed. Adjust seasoning and lemon juice. To serve, garnish with tomato and basil.