The sophomore forward, who scored only four goals and had only 10 points in the first half of the NHL season, broke loose for three goals and two assists during a 10-2 rout of the Boston Bruins on Thursday night.
"There has never been a doubt about his talent," said coach Paul Maurice, whose patience with Steen paid off handsomely. "There were questions about other parts of his game, but he has worked hard to improve those."
Bates Battaglia and Matt Stajan added two goals each and John Pohl, Kris Newbury and Chad Kilger had one each for the Leafs, who moved three points ahead of last-place Boston in the Northeast Division with only its second road win since Nov. 24.
Toronto last scored 10 goals in 1998.
Andrew Raycroft made 14 saves in the first period and 29 in all in earning the goaltending win against his former team.
The Bruins were ashamed after they were manhandled. They lost 5-1 in Toronto on Tuesday and have been outscored 20-3 in losing three in a row. They have allowed 28 goals in their last five games.
"We are embarrassed," said defenceman Brad Stuart. "It was almost like we gave up.
"We weren't mentally or emotionally prepared and you can't go much lower."
Toronto took advantage of Boston's defensive woes while handing the Bruins their worst loss of the season.
After Pohl gave Toronto a 1-0 lead with a power-play goal, Shean Donovan and Wayne Primeau, on a power play, put the Bruins ahead 2-1 in the first period.
Battaglia tied it 2-2 when he knocked in the rebound of an Ian White shot 1:36 into the second.
Steen put the Leafs ahead 3-2 at 8:24 on a great individual effort when he drove toward the net and beat Thomas while falling into the crease.
Toronto pulled away with three goals in 42 seconds - Battaglia at 15:33, Steen at 15:51 and Newbury at 16:15 - to make it 6-2. The splurge missed the club record, set in 1940, by three secones.
The firepower was unexpected given Toronto was missing five regulars with injuries.
"We have to take advantage of the ice time we get with the guys out of the lineup," Battaglia said. "Everybody needs to contribute."
Stajan scored twice in the third and Steen and Kilger closed out the scoring for Toronto.
Pohl, Boyd Devereaux, Tomas Kaberle and White also had two assists.
It was the seventh meeting of the teams. Toronto has outscored Boston 21-7 in its three victories while being outscored 12-4 by Boston in its four losses.
"We've lost our mojo," Bruins coach Dave Lewis said. "This is caused by a combination of breakdowns, but even in the dark days we have to stick together."
Tim Thomas allowed eight goals on 31 shots. Philippe Sauve took over late in the second period and allowed two goals on eight shots.
Notes: On power plays, Toronto was 1-for-5 and Boston 1-for-7 . . . Darcy Tucker was out of the Toronto lineup after returning last game. His left foot, which stopped a puck last weekend, was too swollen to get into a skate boot . . . The Leafs scored five goals in a period for the second time this season . . . The Leafs returned home to face the Buffalo Sabres on Saturday night.