Uhh, with the mall owner. It's in the lease.
There are many concepts and clauses in retail leases. And many remain subject to ongoing negotiations throughout the term.
Starbucks to close Teavana, continue paying underlying lease payments, and list for sublease subject Teavana suites in Simon's malls.
That fulfills ANOTHER lease clause for them to mitigate loss (if any) to Simon, while also seeking to minimize costs and losses for themselves.
That leaves Simon to try forcing Starbucks to remain open, and arguing what amount of damages they claim, if any.
In practice since it is Starbucks, the suites themselves may be a valuable locations and properties, which another retail entity would desire to acquire from either Starbucks or Simon.