The OP needs to open the glove compartment, take out the Owner’s Manual, and find the toll-free phone number for Subaru of America (SOA). Phone SOA, tell the company rep about the stalling problem, as well as the attitude of the dealership, and allow them to work their magic with the dealership.
The car is fully warrantied, and any authorized dealership has to work with you to resolve the issue. If the dealership’s mechanics are not competent to resolve the problem, then SOA can send their regional service representative to instruct the mechanics in what to do–which is probably just a re-flash of the software in the engine control module.
When you speak with the SOA rep, keep the conversation civil, but make sure you communicate that you are very serious about resolving the issue, including filing a Lemon Law complaint if the issue is not resolved promptly. That is what I did (in writing) for a friend who had similar problems with his Toyota. That resulted in Toyota sending both the regional service rep and a Japanese engineer to the dealership, where they personally worked on his Rav-4, and within about 3 hours they found and fixed the problem that had eluded the local mechanics on several previous visits.
If you make it clear to the SOA rep that you are going to do whatever is necessary to resolve this to your satisfaction, they will get the issue resolved for you.