To prepare meat loaf, heat a large sauté pan over medium heat; add spinach and 2 tablespoons water to pan. Cover and steam 2 minutes or until spinach wilts. Remove pan from heat; cool 5 minutes.
Combine ground sirloin, veal, pork, and next 8 ingredients (through egg white) in a large bowl; gently mix just until combined. Add spinach and oats; gently mix until blended.
In a 9 x 13-inch glass or ceramic baking dish coated with cooking spray, shape mixture into a 9 x 5-inch loaf.
To prepare glaze, combine ketchup, brown sugar, and cider vinegar, stirring with a whisk until blended. Brush glaze over meat loaf. Bake at 450° for 15 minutes. Reduce oven temperature to 350°. Bake an additional 45 minutes or until done; let stand 10 minutes before slicing.