Lamah needed just 31 minutes to score a hat trick at Toyota Stadium, setting up Dallas' 6-2 thrashing of Real Salt Lake.
The Belgian opened the scoring in the eighth minute, tapping in a cross from Michael Barrios at the back post.
Lamah doubled the lead midway through the half, finishing calmly after receiving a pass from Maximiliano Urruti.
Just after the half-hour mark, he had his hat trick after being played in behind by Carlos Gruezo.
It was 5-0 at the break for Oscar Pareja's Dallas, Urruti providing a chipped finish before Tesho Akindele also struck.
Omar Holness and Kyle Beckerman pulled goals back for the visitors, but 16-year-old Jesus Ferreira sealed the resounding win for Dallas.
NYCFC had to come from behind to claim a dramatic 2-1 victory over the Philadelphia Union.
Fabrice Picault put the Union ahead with a fine finish in the 69th minute, but the hosts rallied.
Maxime Chanot leveled with a header in the 80th minute and Alexander Callens was the hero after a scrappy goal from a corner.