nuance using present indicative vs subjuctive after desde que

Hola Wendy

I can see why you may take this as a future action but if we read the whole sentence, you can see that it is about a "habitual action" so it's talking about something that happens "every Sunday" , therefore, we consider the subordinate clause something that does happen "from the moment I finish lunch [every Sunday] I [always] study Spanish. 

If we modify the sentence a little: 

El domingo que viene estudiaré español desde el momento en que termine de comer.

Next Sunday I'll study Spanish from the moment I finish lunch.

We are now talking about a future Sunday (next one) and when that day comes I will study Spanish when I finish lunch that day. (present subjunctive: termine)

I hope this clarified the difference. It's a tricky one, the same as when we use "cuando". 

Saludos cordiales

Inma