In large pot over medium heat, combine oil and curry paste. Cook, stirring, for one minute. Add coconut milk, onion, carrots, peppers, green onions, rice vinegar, fish sauce and salt. Bring to a simmer; reduce to low. Cover and simmer, stirring occasionally until vegetables are softened, about one hour. Add spinach and shrimp, stir to combine. Continue to cook until shrimp are pink and cooked through and spinach is wilted, about five to six minutes more.
Serve over steamed rice, if desired.
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