How to Make It
Preheat the oven to 375°. Spread the focaccia on a large rimmed baking sheet. Toast for about 30 minutes, stirring halfway through, until dry and golden around the edges. Transfer to a large bowl.
In a large skillet, melt 2 tablespoons of the butter in the olive oil. Add the fennel and onions and cook over moderate heat, stirring occasionally, until very tender, about 15 minutes. Add the garlic, thyme and fennel seeds and cook until fragrant, about 1 minute. Scrape the fennel mixture into the bowl with the focaccia. Add the stock and toss until the bread is evenly moistened. Season with salt and pepper.
Increase the oven temperature to 400°. Butter a 9-by-13-inch baking dish with 1 tablespoon of the butter. Scrape the stuffing into the baking dish and dot the top with the remaining 1 tablespoon of butter. Cover with foil. Bake for about 20 minutes, until heated through. Remove the foil and bake for about 20 minutes longer, until the top is golden and crisp. Serve hot.