Ingredients
4 tablespoons butter1/2 cup chopped onion1 teaspoon ground mustard1 teaspoon salt1/2 teaspoon coarsely ground pepper2 garlic cloves, minced1/4 cup all-purpose flour2 cups whole milk3 blocks (4 ounces each) sharp cheddar cheese, shredded and divided 1 block (4 ounces) Monterey Jack cheese, shredded3 pounds russet potatoes, peeled and thinly sliced3/4 cup dry bread crumbsMinced chives, optional
Preparation
Preheat oven to 350°. In a Dutch oven, melt butter over medium heat. Add onion, mustard, salt and pepper; cook until onion is tender, 6-8 minutes. Add garlic; cook until fragrant. Whisk in flour; continue whisking 3-5 minutes.
Whisk in milk; bring to a boil. Reduce heat; simmer, uncovered, until thickened slightly, 8-10 minutes. Gradually stir in 2 cups shredded cheddar cheese; stir in Monterey Jack cheese. Add potatoes and toss to coat. Simmer 10 minutes, stirring frequently.
Transfer potato mixture to a greased 13x9-in. baking dish. Top with remaining cheddar cheese. Bake, uncovered, for 1 hour. Top with bread crumbs; return to oven until potatoes are tender, 10-15 minutes. Let stand 15 minutes before serving. If desired, sprinkle with chives.
