In a small bowl, combine the softened butter, sun-dried tomato, scallion and garlic; season with salt and pepper.
In a large saucepan, add the potatoes and enough salted water to cover. Cover and bring to a boil, then lower the heat and simmer until tender, about 15 minutes. Drain, then smash the potatoes with 1 tablespoon butter; season with salt and pepper.
Season the fish with salt and pepper, then coat lightly with the flour. In a large nonstick skillet, melt the remaining 2 tablespoons butter over medium heat. Add the fish and cook, turning once, until golden and firm to the touch, 7 to 8 minutes total.
Transfer the fish to 4 serving plates and top with the tomato-scallion butter. Serve with the smashed potatoes.