Stephen Myler's precise kicking opened up a 20-0 lead but when Guilhem Guirado's crossed on the stroke of half-time, a comeback looked on the cards until Saints' steely determination closed the game out.
Jim Mallinder's men will face off against Leinster in Cardiff after the Irish Province stormed past defending champions Toulouse on Saturday, but 25-year-old Hartley has full faith that his teammates can bring home Europe's biggest prize for the first time in 11 years.
"To go to Cardiff is special - it's a big occasion," said Hartley.
"I've got a great team around me and it makes my job easier when you have got 14 guys around you all doing their job.
"When points are on offer teams are going to take them, we knew they had a good kicker and knew we couldn't give away any stupid penalties and I think we achieved that."
"For the first 40 minutes no-one was moving in the scrums, but that hard work in the first half paid off later on and it started to go our way in the second half."
"I'm just happy to go to Cardiff and in knock-out rugby this team can win any game on its day."