I haven't seen Sidney, so I'll just say that often people feel their Boxers are too skinny, when they're really just right. At her ideal weight, you should be able to just see the outline of her last 3 ribs, and when you feel her ribs they should feel like the back of your hand.
If she's too skinny (her spine and hip bones are prominent when she's standing) increase her food intake. Hamburger and rice will only help while you're feeding it, as you've found. If she's getting 1 c. of food, up it to 1 1/4 c. If she's still too skinny after 1-2 weeks, add another 1/4 c. You do want the weight to come on slowly. Once she gets to her ideal weight, be sure to cut back slightly or she'll keep gaining.