Adas, the others have it nailed.
But once you Rasa knows her "sit before meal" thing really well, what you CAN do (to challenge her, because we know that GSDs need to be challenged) is you can look for offered behavior.
That's when your dog has been asked for the same thing often enough that she should OFFER the behavior before you ask for it, or even when you don't ask for it at all.
It sounds to me like that's almost what you're trying to do here. The thing is, it's just a bit early for that. She needs probably another month of consistent Sits (with the verbal command) before dinner. You just want to make sure she is absolutely clear what you want.
Once she has that down, then you should be able to just hold up the bowl and she'll probably offer it. If she doesn't, luring her into a sit (by moving her bowl slightly over her head like we would lure a puppy with a treat into a sit) will do it. Plop! She'll sit for you!
Offered behaviors are wonderful because after a while, your dog will be THINKING (ah! We're working her brain again!) "what does my owner want in this situation?" Some behaviors, like sitting before dinner will just become automatic then (no thinking required). But Rasa will have started the process of thinking "what does my owner want?" in other aspects of her life.
And soon enough, you'll end up with a dog that seems like she reads your mind. She sits when you want her to, waits when you want her to, lies down when it seems most reasonable to.
And life is just really smooth together.
But it's part of an evolution.
You're at the first step.
Don't rush it. Don't confuse her yet. Talk to her. Like Gunnersmom, I have conversations with my dogs. but when I'm giving them commands, there is no doubt in their minds "this is a command." I start with their name. I give the command in a clear firm voice. And I watch for immediate reaction.
The reward is sometimes a treat and always a Good "Command!" (good sit, good down, good come) to reinforce that they did what I wanted. They aren't a good boy or good girl just for existing. But what they did was exactly what I wanted: "Good Wait!" (This is important especially when they offer behaviors).
It sounds like you and Rasa are making great progress. Keep it up!