Zucchini Gratin with Gruyère and Panko Breadcrumbs - Yay! For Food

Ingredients

• 2 lb. (3-4) zucchini, sliced into ¼ inch pieces
• 1 large bell pepper, sliced
• 1 large onion, sliced
• 1 tbsp (½ oz) unsalted butter
• Salt and pepper, to taste
• 4 tbsp (2 oz) unsalted butter
• 4 tbsp all-purpose flour
• 2½ cups milk (I used 2%), room temperature
• ¼ tsp ground nutmeg
• 1 cup Panko breadcrumbs, divided
• 1 cup (2 ½ oz) Gruyère cheese, shredded