Heat 2 tablespoons olive oil in a Dutch oven or large pot over medium-high heat. Add the onion, celery and garlic and cook, stirring occasionally, until the vegetables soften, about 5 minutes. Add the tomatoes, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until thickened, about 8 minutes.
Add 1/2 cup water, the wine and red pepper flakes to the pot and bring to a simmer. Add the mussels and tilapia; cover and cook until the mussels open and the tilapia is cooked through, about 5 minutes. (Discard any unopened mussels.) Season with salt and pepper. Gently stir in the parsley.
Meanwhile, preheat the broiler. Arrange the bread on a baking sheet and brush with the remaining 1/2 tablespoon olive oil. Broil until toasted, about 3 minutes. Serve with the seafood stew.