Mussels - Richmond Hill/Markham
I'm not sure about Mussels in Mushroom/Sherry sauce, but some Mussels that I have quite liked have actually come from the Pickle Barrel at Markville Mall. Not all Pickle Barrels have Mussels - just the newer upscale ones.
I've only had the Spicy Tomato one but I find it quite delicious. Inexpensive too I think at $5.99 - My girlfriend and I sometimes each order one of these and share a salad for a nice light meal.