Heat oil in a large saucepan over medium-high heat. Add onion; cook, stirring occasionally, until just beginning to brown, about 6 minutes. Add jalapeño, garlic and ginger; cook, stirring often, until fragrant, about 1 minute. Add tomato paste, cumin, coriander and turmeric; cook, stirring constantly, until fragrant and the vegetables are coated, about 30 seconds.
Stir in water and coconut milk; bring to a vigorous simmer over high heat. Reduce heat to medium and add sweet potatoes; bring to a gentle boil. Cook, stirring occasionally, until the sweet potatoes are tender, about 15 minutes.
Add beans, spinach and salt; stir until the spinach is wilted, about 1 minute. Remove from heat and stir in lemon juice.
