With American Airlines. The long leg is operated by a 787 or 777 with lie flat seats.
The short leg from Montreal to the US is in Economy class only.
How to book
The cheapest Business class tickets can be bought via OTA's. Link with prices below.
Those prices are available for departures from March to May 2021.
TRAVEL FROM TURNAROUND MUST COMMENCE NO EARLIER THAN 24 HOURS AFTER ARRIVAL AT TURNAROUND.
TRAVEL FROM LAST STOPOVER MUST COMMENCE NO LATER THAN 12 MONTHS AFTER DEPARTURE FROM FARE ORIGIN.