Combine the ground ham, ground pork, onion, oats, eggs, parsley, ground mustard, pepper, and evaporated milk in a large bowl. Mix well. Form in to 2 loaves; place into 9x13 inch baking pans.
Bake loaves in preheated oven for 30 minutes. Stir the brown sugar and vinegar together in a small saucepan; boil for 1 minute. Remove loaves from oven, and pour brown sugar mixture over. Return loaves to oven; baste frequently with the pan juices until browned and cooked through, 30 to 45 minutes longer.
Combine the mayonnaise, sour cream, horseradish, and lemon juice in a small bowl. Season to taste with salt and pepper. Serve sauce over sliced ham loaf.