Preheat oven to 450 degrees. Butter an 8 by 8-inch baking dish with 1 tablespoon butter. In a large pot of salted boiling water, cook fennel, covered, until tender, about 15 minutes. Place fennel cut-side down to a paper towel lined plate and drain 5 minutes.
Transfer fennel, cut-side up, to prepared pan. Dot with 1 tablespoon butter; season with salt and pepper, and top with thyme and parmesan. Bake until golden brown, about 20 minutes.