The BEST Keto Green Bean Casserole

Ingredients

• 1/2 medium onion, thinly sliced
• 1 teaspoon baking powder or 1 tablespoon cornstarch
• 1/4 cup pork panko
• 1/2 teaspoon salt
• nonstick cooking spray
• 1 lb fresh green beans, ends trimmed & halved
• 2 tablespoons unsalted butter
• 10 oz white button mushrooms, quartered
• 1 teaspoon salt, divided
• 1/4 teaspoon black pepper
• 2 cloves garlic, minced
• 1/4 teaspoon ground nutmeg
• 1 1/2 teaspoon arrowroot powder
• 1 cup chicken or vegetable broth
• 3/4 cup heavy whipping cream