For sauce: Preheat oven to 325°F. Line rimmed baking sheet with parchment paper. Combine tomatoes, basil leaves, oil, vinegar, and sugar in medium bowl; toss to blend. Spread tomato mixture in single layer on prepared sheet; sprinkle with salt and pepper. Roast until tomatoes are slightly shriveled, about 40 minutes. Transfer tomato mixture to medium saucepan. Add clam juice, wine, saffron, and crushed red pepper; bring to boil. Reduce heat and simmer 10 minutes to blend flavors and reduce slightly. Add butter and stir until melted. Season sauce with salt and pepper. (Can be prepared 4 hours ahead. Rewarm over medium heat before serving.)
For fish: Preheat oven to 425°F. Line another rimmed baking sheet with parchment. Arrange fish on parchment; brush fish with oil. Sprinkle fish with thyme, lemon peel, salt, and pepper. Roast fish, without turning, until just opaque in center, about 10 minutes for monkfish and 6 minutes for halibut.
Spoon Celeriac Mash onto 4 plates. Top each with 1 fish fillet. Spoon sauce over. Garnish with basil sprigs, if desired.