In a large pot over medium heat, add the oil, scallions, celery, and ½ teaspoon salt. Cook until softened, about 3-5 minutes, stirring occasionally.
Add the water, beans, and remaining 1 teaspoon salt. Increase the heat to high to bring to a boil. Once boiling, remove about 1 cup of the liquid, getting some beans in there to help thicken it up, and transfer to a high speed blender. Stir the tortellini into the boiling water and cook according to the package instructions (usually around 2-4 minutes). Turn off the heat.
In the blender, add the spinach and miso paste and blend until totally smooth. Pour back into the pot and stir in your lemon juice. Taste and add more salt or lemon, if needed.
Serve topped with feta, black pepper, fresh herbs, and a drizzle of olive oil.