Heat oven to 425°. Spread pecans on a baking sheet. Place in oven to toast until fragrant, about 6 minutes. Set aside.
Melt 2 T butter in a small skillet. Cut cornbread into 2"-thick slices. Place, cut side down, on a 17x12" baking sheet. Brush tops with melted butter. Toast cornbread until golden brown, 10 minutes. Transfer to a wire rack to cool.
Heat remaining 4 T butter in a medium skillet on med-high. Add shallots and celery. Cook, stirring occasionally, until softened, about 3 minute
Add mushrooms, thyme, rosemary, salt, and pepper; cook until slightly softened, about 3 minute
Add white wine; continue cooking until absorbed, about 1 minute. Add cream. Cook 30 seconds; remove pan from heat.
Crumble cornbread into a large bowl. Add mushroom mixture and toasted pecans; toss.