Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish; set aside.
Spread the bread evenly on a baking sheet and bake, tossing once or twice, until toasted, 10 to 15 minutes.
Meanwhile, melt the butter in a large saucepan over medium heat; set aside 2 tablespoons. Add the fennel to the saucepan and cook, stirring occasionally, until tender, 20 to 25 minutes.
In a medium pot over medium-high heat, combine the chicken broth, sun-dried tomatoes, vinegar, oregano, salt and pepper. Bring to a simmer and cook for 10 minutes.
In a large bowl, toss the toasted bread with the fennel and the broth mixture until the bread is coated and has fully absorbed the liquid. Put the stuffing in the prepared baking dish, cover with foil and bake for 20 minutes. Uncover the stuffing and brush it with the reserved 2 tablespoons melted butter. Bake, uncovered, until browned on top, about another 20 minutes. Garnish with fennel fronds and serve hot.