Place a 10-in. cast-iron skillet in the oven; preheat to 425°. Whisk together first 5 ingredients. In another bowl, whisk milk, eggs, 1/4 cup olive oil, honey and vinegar; whisk into dry ingredients. Remove skillet from oven; lightly grease with remaining olive oil. Pour in batter. Bake until golden brown, about 15 minutes. Cool 10 minutes; remove from pan to a wire rack to cool completely. , Meanwhile, in a large cast-iron skillet, cook sausage over medium-high heat, crumbling meat, until no longer pink. Remove and drain. In same skillet, cook mushrooms, celery and onion until onion is tender, about 5 minutes. Crumble cornbread into a large bowl; stir in sausage, mushroom mixture and remaining ingredients. Transfer to a greased 13x9-in. baking dish. Refrigerate, covered, at least 8 hours. , Remove from refrigerator 30 minutes before baking. Preheat oven to 375°. Bake, uncovered, until top is browned and mixture is set, 40-45 minutes.