TheFork users who ate in Essen have said really good things about LUKAS kulinarischer Bahnhof, Müllers auf der Rü and Am Kamin. Maybe you should give them a try!
Some of the more popular German restaurants having a terrace in Essen according to TheFork users include:
LUKAS kulinarischer Bahnhof, with a 9.2 rating
Am Kamin, with a 9.6 rating
Restaurant Mausefalle, with a 9.4 rating