When a tenant breaks a lease, the landlord has an obligation to mitigate their losses and try to re-rent the unit as soon as possible. If the landlord incurs any rent losses and extra expenses for advertising the unit, the landlord would have to sue the former tenant in Small Claims Court if the tenant vacated prior to September 1, 2021. https://www.attorneygeneral.jus.gov.on.ca/english/courts/guides/Guide_to_Making_a_Claim_EN.html
As of September 1, 2021, a landlord can file an application for loss of rent that the landlord incurred with the LTB up to one year after the tenant moves out of the rental unit. The tenant must have moved out of the rental unit on or after September 1, 2021. However, in order to file this application with the LTB, you will have to find out the tenant's new residential address to be able to serve them. The prescribed form is available on the LTB website at https://tribunalsontario.ca/ltb/forms/.