Preheat the oven to 425 degrees F.
Cut the melons in half lengthwise and scoop out the pulp and seeds to create a pocket on the inside of each half. Set the melon aside.
Put 2 tablespoons grapeseed oil in a large skillet over medium-high heat, add the onion and ginger and cook for 4 minutes, or until the onion is soft and golden. Add the tomato, curry leaves, garam masala, mustard seeds, ground turmeric and season with salt. Cook the mixture for 5 minutes, stirring occasionally. Let cool slightly and then spoon into bitter melon halves. Put the halves back together and tie with some kitchen string.
Put the remaining 2 tablespoons grapeseed oil in a clean skillet and place over medium-high heat. Brown the stuffed melons on all sides and then place them on a baking sheet. Bake until the melons are tender, about 15 minutes.
NotesTo remove excess bitterness from the melons, generously salt each side of the cut melons and let sit in a colander for 15 to 20 minutes. Rinse the melons well to remove all the salt and then continue with the recipe.