Preheat the oven to 250 degrees F. Combine 2 cups water, the rice, ginger and 3/4 teaspoon salt in a large pot. Bring to a boil, then stir and reduce the heat to low. Cover and cook until the liquid is absorbed and the rice is tender, 16 to 18 minutes. Remove from the heat and add the broccoli (do not stir). Cover and set aside to let the broccoli steam.
Meanwhile, heat 2 inches of vegetable oil in a separate large pot or Dutch oven over medium heat until a deep-fry thermometer registers 375 degrees F. Whisk the flour, mustard powder, 1/4 teaspoon salt and the seltzer in a large bowl until smooth. Season the cod with salt. Working in batches, dip the cod in the batter, allowing the excess to drip off, and fry until golden brown and crisp, 6 to 7 minutes. Remove to a wire rack set over a baking sheet and season with salt; transfer to the oven to keep warm. Return the oil to 375 degrees F between batches.
Gently fluff the rice with a fork. Discard the ginger and stir in the sesame oil and scallions; season with salt. Serve the cod with the chili sauce, broccoli rice and lemon wedges.