Zucchini and Cheese Gratin - Spread the Mustard

Ingredients

• 1 cup panko bread crumbs
• 2 Tbsp grated parmesan cheese
• 1 lemon, zested
• pinch salt
• 6 Tbsp salted butter
• 1 Tbsp canola oil
• 2 shallots, diced
• 2 garlic cloves, minced
• 3 Tbsp all-purpose flour
• 1 ¼ cups whole milk
• 1 ½ cups grated firm cheese such as Gruyère, sharp cheddar, aged havarti
• 1 Tbsp Dijon mustard
• ½ tsp salt
• ¼ tsp black pepper
• 2 lbs zucchini (about 8 small), sliced into 1/4-inch thick rounds