I'm not a huge fan of Belgian beer-- if you are, then you'll want to go here for that alone -- but we love to visit for the mussels. They are always a good size, properly cleaned (no pebbles of sand), properly cooked (tender, not chewy or falling apart), and flavorful. You have a choice of 8 different broth preparations. I haven't had one yet that I haven't liked, but our favorites are the Provence (leeks, garlic, bell pepper, mushrooms and more in a light cream broth) and my new favorite, the Ghent (caramelized leeks, garlic, blue cheese and bacon). Before you're done, use every bit of the roll to dunk chunks of bread in the broth and soak up the yummy goodness.
Every bit the equal of the mussels are the fries that come with them. They're cut thin like shoestrings, but shorter, and taste great all alone -- but the highlight is the dipping sauce, which will have you scraping the bottom of the dish for every last bit. The sauce is actually mayo-based, infused with bourbon -- but has enough kick that you'd think it was mustard-based. Just trust me that the taste is better than it may sound.
And there's actually plenty more to choose from: they make a great burger (choice of toppings, but I recommend the eponymous Monk's burger, with caramelized leeks and blue cheese -- yum!), ad have some really good salads (the duck salad is our favorite). The problem is that we don't get here often, and the mussels & fries are the reason we come, so the other food options often lose out! On our latest visit, we shared a duck salad for appetizer, and then shared two pots of mussels, for a thoroughly enjoyable meal.
Do read the comments about how crowded this place gets. We were able to time this visit for an early dinner at 5 pm on Sunday, when there were empty tables and we could be seated immediately. We used to come at prime dinner times, but found the longs waits (and lack of adequate space to stand while waiting) less-than-optimal, so now we just come when we can time it for off-prime time visits.
And going back to the beer, the servers are very knowledgeable, and will help you find a beer you like. I find many of the Belgian beers too sweet, and I asked for something neither too hoppy nor too sweet, and got a very nice malty St. Bernardus that I enjoyed.