Cook the rice according to package directions. Keep warm.
Meanwhile, add the lemongrass and ginger to a food processor or blender. Pulse 4 or 5 times to chop. Add the onion, garlic, and curry powder. Process until a fragrant yellow paste is formed, scraping down the sides as needed.
Heat the oil over medium in a heavy-bottom pot.
Add the curry-onion paste and cook, stirring, for a few minutes, or until the raw smell is gone. Add the carrots and stir. Add the coconut milk and water and turn the heat up to medium-high. Once simmering, lower the heat and let simmer for 10 minutes.
Add the fish sauce and bok choy and stir. Cover and simmer for a few minutes, or just until the greens are wilted. Add a little more water if needed.
Season the tilapia with salt and pepper and add to the curry. Stir and cook for 1 minute. Add the shrimp and stir and cook just until the shrimp and fish are opaque and cooked through, about 2-3 minutes.
Remove from the heat and let sit for about 5 minutes. Taste for seasoning. Serve with the rice and a big wedge of lime.
