Heat oil in a medium saucepan over medium heat. Add coriander and cumin, and cook until fragrant, 1 minute. Raise heat to medium-high, and add turmeric, chiles, shallots, lemongrass, garlic, ginger, scallions, lime zest, and lime juice. Cook until softened, 3 to 4 minutes.
Add stock, salt, and pepper. Bring to a simmer, and cook 3 minutes. Pour mixture through a fine sieve into a clean saucepan; discard solids. Bring broth to a gentle simmer. Add salmon, and cook to desired doneness, 2 to 3 minutes for medium-rare. Stir in spinach and herbs. Serve immediately.