In a large saucepan, combine diced onion, frozen peas, and frozen spinach along with tarragon. Pour in 4 cups of water or vegetable broth and heat over medium-high until simmering, about 5 minutes. Then reduce heat to medium-low and cover. Cook for about 20 minutes until tender.

Remove from heat and blend the mixture until smooth using an immersion blender. Transfer to a countertop blender if necessary.

Stir in sherry for depth of flavor, then add cream or half and half, followed by curry powder if using. Warm gently without boiling.

Taste the soup and adjust seasoning with salt if needed. Add more broth for thinner consistency.

Serve warm or chilled, garnishing with fresh herbs or a dollop of cream before enjoying.