Cooked lettuce absorbs the broth's flavor and makes a silky side dish -- all in one pot.
In a large pot, melt butter over medium. Add shallots and cook, stirring occasionally, until softened, 10 minutes. Add lemon zest and broth and bring to a simmer. Season salmon with salt and pepper and add to pot. Cover and cook 3 minutes. Add lettuce (pot will be full), cover, and cook until lettuce is wilted and salmon is cooked through, 9 minutes. Sprinkle with lemon juice and season with salt and pepper. Serve topped with dill and broth if desired.
Cooking lettuce intensifies its natural sweetness.