For sauce: Bring first 4 ingredients to boil in medium saucepan. Reduce heat, cover, and simmer until cauliflower is very tender, about 17 minutes. Cool slightly. Transfer mixture to blender; add 3 tablespoons water and puree until smooth. Season with salt and pepper. (Can be made 1 day ahead. Cover; chill. Rewarm over medium heat, stirring, and adding more water to thin sauce, if desired.)
For lentils and browned butter cauliflower: Heat oil in medium saucepan over medium heat. Add onion, carrots, and celery; sauté until soft, about 8 minutes. Add lentils, curry, and paprika; stir 1 minute. Add 2 1/2 cups water; bring to boil. Reduce heat, cover, and simmer until lentils are tender, stirring occasionally, about 30 minutes. Uncover; stir until liquid is absorbed, about 5 minutes.
Meanwhile, cook butter in large nonstick skillet over medium-high heat until browned, about 2 minutes. Add cauliflower; sauté until beginning to brown, about 5 minutes. Add 1/4 cup water; cover and cook until cauliflower is crisp-tender and water evaporates, about 3 minutes longer. Stir in lentil mixture. Season with salt and pepper. (Can be made 2 hours ahead. Let stand at room temperature. Rewarm before serving.)
For gremolata: Mix parsley and lemon peel in bowl. Season with salt and pepper.
For fish: Sprinkle monkfish with salt and pepper. Heat oil in large nonstick skillet over medium-high heat. Add fish; sauté until just opaque in center, about 6 minutes per side.
Divide sauce among 4 plates. Spoon lentil mixture alongside. Place fish atop lentils; sprinkle with gremolata.
*Available at specialty foods stores and some supermarkets.