Add stock, water, and coconut milk; bring just to boiling. Add soba noodles; return to boiling. Reduce heat and simmer, uncovered, about 8 minutes or until noodles are tender, stirring occasionally. Add shrimp and green onion tops. Cook for 1 minute to heat through. Remove from the heat and stir in spinach, soy sauce, and lemon peel (if using).