The Roompot-Charles rider emerged the fastest after a 133km long break to finish ahead of Filippo Fortin (Cofidis) and Jonas van Genechten (Vital Concept-B&B Hotels) in a downpour.
The trio just managed to hold off the peloton after their two-minute lead in the final 30km was cut to nothing at the line.
“This is definitely the biggest victory of my career and I still can’t believe it. It’s really awesome and I will never forget today," Asselman said.
"The plan was to get in the break and go for the best climber’s jersey. On the climb, I had no explosivity in my legs though so was angry.
"I kept pushing out of frustration but I thought we’d get caught because the gap was going down really fast in the last 50km. We kept riding really hard and everyone worked well together.
"In the last few kilometres, I realised it might be possible and gambled to start my sprint at the right moment. I went full gas and looked under my elbow near the line and saw there was a big gap behind and I couldn’t believe I was going to win."