Heat the oil in a wide, deep skillet or wok over medium heat. Add the mustard seeds and let them pop about 15 seconds, then add the curry leaves.
Add the onion and cook 5 to 6 minutes, stirring occasionally, until soft and lightly golden at the edges.
Stir in the garlic and ginger and cook 1 minute. Add the coriander, turmeric, and chili powder, and stir 30 seconds.
Add the chopped tomato and cook 2 to 3 minutes, mashing slightly, until it breaks down into the spices.
Pour in the coconut milk and add the kohlrabi cubes. Bring to a gentle simmer, cover, and cook 10 to 12 minutes until the kohlrabi is fork-tender.
Season with salt, raise the heat slightly, and add the squid. Cook uncovered for exactly 2 minutes, stirring gently, until opaque and curled.
Remove from heat immediately, stir in the lime juice, and scatter cilantro on top before serving.