Cheesy Broccoli Bake

Ingredients

• 2 pounds fresh broccoli, trimmed and cut up
• 2 tablespoons unsalted butter, plus more for greasing the casserole
• 1/4 cup chopped celery
• 1/4 pound fresh mushrooms, sliced
• 1/4 cup chopped onion
• 1 (8-ounce) can sliced water chestnuts
• 1 (10 3/4-ounce) can condensed cream of mushroom soup
• 1/2 pound cheese product, softened
• 1/2 teaspoon garlic salt
• 1/4 teaspoon pepper
• 1 cup grated Cheddar