Stir together the hash browns, 1 1/2 cups of the cheese, and the onion, salt, pepper, eggs, and sour cream.
Cook the sausage in a large skillet 10 minutes or until lightly browned. Remove from the skillet with a slotted spoon, reserving the drippings in the pan. Add the flour to the pan and cook for 1 minute, stirring constantly. Add the tomatoes with juice and cook 5 minutes, or until thickened. Reduce the heat to low and stir in the cream. Season to taste with salt and pepper. Set aside, keeping warm.
In a large bowl, stir the sausage into the hash brown mixture and transfer to a lightly greased 13-by-9-inch baking dish. Top with the remaining 1/2 cup cheese. Bake until just set and golden brown, 30 minutes.
Spoon the casserole onto the serving plates; top with the warm tomato gravy and a sprinkling of chives, if desired.