Serwa, the reigning world champion from Kelowna, B.C., came from fourth to win gold in the final of the World Cup event after Switzerland's Katrin Mueller and French veteran Ophelie David tangled behind her.
Simmerling, whose best finish since switching from alpine to ski cross was ninth at San Candido last year, just avoided the collision and wound up second.
"I saw Katrin's skis go up in the air and she took a tumble and almost decapitated me with her ski," said Simmerling, who hails from West Vancouver, B.C. "And I had to ski around her along the net. The conditions were pretty tough, but I think the race was safe."
Simmerling finished third but was bumped up when David was disqualified after a review. Mueller took bronze.
For Serwa, it was her first victory since winning both races here in 2011. She has been trying to build her confidence from race to race after returning from a knee injury that ended her season last January.
"It was fun day and I'm happy to be back on top," said Serwa. "It feels like it's been a really long time. The pieces are starting to fall together."
Serwa dominated her quarter-final and semifinal, but it was the first time she had to battle from fourth to first in a final.
"It almost feels like I worked for it more this way than just getting in front and staying in front," said Serwa. "I had to be smart and had to be tactical, rather than just skiing."
The silver was a welcome surprise for Simmerling, who had yet to even reach a World Cup quarter-final this season. She advanced from the semis after contact between Norway's Marte Hoeie Gjefsen and France's Marielle Berger Sabbatel.
"I'm obviously very excited and little surprised," said Simmerling. "I tried to ski smart and did my best and I think luck was on my side, but that's the sport."
Serwa is third in the ski cross standings with 195 points behind Fanny Smith of Switzerland (336) and Gjefsen (207).
Smith had opened the season with a run of three straight victories, but she was defeated by Mueller in Sunday's semifinal.
On the men's side, Calgary's Brady Leman failed to reach the podium for a third straight time and finished fifth. He has two silvers so far this season.
"It was a tough race today," said Leman. "I made a lot of good passes and was racing really hard to be in the finals again. In the semis I was in second place when another racer made a very aggressive inside pass over a negative turn. He clipped my tips and it spun me out, causing me to crash and miss out on the finals."
Switzerland's Alex Fiva got his first victory this season, finishing ahead of Germany's Daniel Bohnacker and Sweden's Victor Oehling Norberg.
Louis-Pierre Helie of Berthierville, Que., finished eighth.
In ski cross, four skiers race each other down a course of jumps and bumps.