For travel through March 31 (with slightly higher prices in April) Star Alliance member and United partner Air Canada has fares from Denver to Paris and Brussels from $321 roundtrip.
28 day advance purchase is required, along with a 7 day minimum stay, and these fares come with a $300 change fee.
You’ll see one-stop itineraries showing up for $367:
These are $38 base fares in each direction.
You can actually book this even cheaper, found using airfare metasearch site Momondo:
(HT: Airfare Spot)