Mushroom-Spinach Stuffed Onions | America's Test Kitchen

Ingredients

• 5 slices hearty white sandwich bread , torn into pieces
• 4 yellow onions (3 1/2 to 4 inches each)
• 8 ounces cremini or white mushrooms , chopped
• 5 tablespoons unsalted butter
• 1 cup low-sodium chicken broth
• 1 (10-ounce) box frozen chopped spinach , thawed and drained
• 1/4 teaspoon grated lemon zest
• 3/4 cup grated Parmesan cheese
• Salt and pepper
• 1 teaspoon sugar