Bring broth, lemon, garlic, thyme, bay leaf, and 1/2 tsp. pepper to a boil in a large pot. Reduce heat and add escarole and clams. Cover pot; simmer until clams open, about 15 minutes.
Heat oil in a large frying pan over medium-high heat. Add pancetta and cook until crisp, about 4 minutes. With a slotted spoon, transfer pancetta to a paper towel.
Sprinkle fish with salt and remaining 1/2 tsp. pepper. Lay fish in pan and cook over medium heat until bottom edge looks browned, 3 to 5 minutes. Add butter, then turn fish over and cook until opaque in center, about 3 minutes.
Spoon escarole and clams into 4 serving bowls, pouring in just enough broth to cover. Top each with a fish fillet, a few pieces of fried pancetta, and a few parsley leaves.