Pat the fish dry and season with salt. Heat oil over medium-high heat. Sear for about 2 minutes per side, until lightly browned. Transfer to a plate.
Lower the heat to medium. Add more oil if needed. Cook ginger for about a minute, then add onion and garlic. Cook until softened and lightly golden.
Add the shrimp paste and cook for a minute.
Add 1½ cans coconut milk. Once it starts bubbling gently, lower the heat to medium-low and cook for a few minutes.
Add eggplant and chilies. Cook for 8–10 minutes, until the eggplant is tender. Add more coconut milk or a little water as needed.
Add mustard greens and gently return the fish to the pan. Cook for about 5 minutes, until the fish is cooked through. Add watercress near the end and cook just until wilted.
Season with fish sauce, salt, and pepper. Turn off the heat.
Video
Notes
Fish: Choose firm, meaty fish like tangigue, tilapia, bangus, pompano, or snapper. Cod also works, but handle it gently since it flakes easily.
Vegetables: Use eggplant and leafy greens. Sitaw, squash, or sayote also work.
Shrimp paste: Raw or sautéed works. Substitute fish sauce and adjust salt to taste.
Coconut milk: Use full-fat. Start with 1½ cans and add more as needed.
Chilies: Leave whole for less heat, slice for more, or omit.
Lemongrass: Optional. Crush lightly, add with the ginger, and remove before serving.