Hash Brown Breakfast Casserole with Tomato Gravy

Name: Hash Brown Breakfast Casserole with Tomato Gravy
Ingredients: 1 (32-ounce) bag frozen diced or shredded hash brown potatoes; 1 (8-ounce) block sharp Cheddar cheese, grated (about 2 cups); 1 small sweet onion, chopped (about 1/2 cup); 1 teaspoon kosher salt; 1/2 teaspoon black pepper; 3 large eggs, beaten; 1 (16-ounce) container sour cream; 1 (16-ounce) package ground pork sausage, roughly chopped; 3 tablespoons all-purpose flour; 2 (14.5-ounce) cans petite diced tomatoes, undrained; 1/2 cup heavy cream; Garnish: chopped fresh chives