Cheesy Potato Casserole

Name: Cheesy Potato Casserole
Ingredients: 1/4 cup butter , plus more for the casserole dish; 1 yellow onion, diced; 3 cloves garlic; 1/4 cup all-purpose flour; 3 cups vegetable or chicken stock; 2 cups grated cheddar cheese; 1/2 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 30 ounces frozen hash browns; 3 serrano peppers, sliced; 2 tablespoons olive oil; Course salt