Heat the oil in a saucepan over medium-high heat. Cook the onion, serrano pepper, garlic, and curry leaves in the oil until the onions are golden brown. Stir in the curry powder, fenugreek seeds, turmeric, and salt; cook another 3 minutes. Add the green beans to the mixture and stir until evenly coated. Reduce heat to medium-low and cook until the beans are al dente. Pour in the coconut milk and simmer at least 5 minutes more. Remove from heat and stir in the lime juice just before serving.