Heat oven to 375°F. Gently simmer chicken, poaching water, and chicken salt in saucepan; cover and cook 20 to 25 minutes to at least 165°F. Drain; shred into short pieces.
Meanwhile, brown mushrooms in oil in large skillet 9 to 11 minutes until dry. Boil fusilli with tablespoon salt to 3 minutes short of package time; reserve 3/4 cup pasta water and drain.
Add 2 tablespoons butter and garlic to mushrooms; stir 30 seconds. Add cream, milk, sauce salt, and pepper; gently simmer 2 minutes.
Remove from heat; add Parmesan. Fold in pasta, chicken, and 1/2 cup reserved water. Transfer to deep 9-by-13-inch dish; foil-cover.
Bake 20 minutes. Melt remaining tablespoon butter; mix with panko. Uncover; scatter crumbs over.
Bake uncovered 8 to 10 minutes until crumbs golden and center 165°F. Rest 5 minutes; add remaining warm pasta water around edges only if sauce dry.