Spinach Gratin | Umami

Ingredients

• 3 Tbsp unsalted butter
• 1 large yellow onion, finely diced
• 1 clove garlic, minced
• 1/4 cup all-purpose flour
• 2 1/2 cups milk
• 2 tsp kosher salt
• 1/8 tsp nutmeg
• 5 10 oz boxes spinach, chopped, defrosted
• 1/2 cup Parmesan cheese, grated
• 1/2 tsp pepper
• 1/2 cup Monterey Jack cheese, or Gruyere cheese, shredded