Heat the oil in a large saucepan. Add the mustard seeds, let them sputter and crackle, then add the shallots, ginger, jalapeno, and curry leaves, if using. Season with salt and pepper and saute for a couple of minutes until the shallots begin to soften.
Add the bell peppers and mushrooms and saute another 3-4 minutes until the vegetables appear to have softened. Add the turmeric, garam masala, and paprika if using. Stir to mix, then add half the coconut milk and bring everything to a boil over medium-high heat. Cook for a couple more minutes or until the vegetables are done to your liking.
Add the remaining coconut milk, salt if needed, and warm through without bringing to a boil. Turn off the heat and add the lemon juice, cherry tomatoes and spring onions.
Serve hot or warm with rice.