Recipe: Kelsey Barnard Clark's Zucchini Gratin | Williams Sonoma

Ingredients

• 4 zucchini, about 2 lb. (910 g) total, thinly sliced
• Kosher salt and freshly ground black pepper
• 12 Tbs. (1 1/2 sticks) (6 oz./165 g) unsalted butter
• 3 onions, thinly sliced
• 1/4 tsp. freshly grated nutmeg
• 2 Tbs. mushroom powder
• 2 Tbs. all-purpose flour
• 1 cup (8 fl. oz./240 ml) cream of mushroom soup
• 2 cups (5 oz./160 g) grated aged Gouda or Parmesan cheese
• 1 bunch green onions, white and green portions, chopped
• 1 bunch fresh flat-leaf parsley, leaves chopped
• 1/2 cup (1 oz./26 g) chopped fresh tarragon
• 1/4 cup (1/4 oz./5 g) chiffonade fresh basil
• 1 cup (2 oz./60 g) panko
• 1 cup (3 3/4 oz./115 g) grated Gruyère cheese
• Pinch of red pepper flakes