I agree with you. I would go with a fresh sauce on the side, tomato or maybe roasted red pepper sauce. If you really wanted to dress it up, I think that smoked salmon would too work well, like blini on steroids.

Kristen W, agree with you - was thinking bright and sharp flavors when I read your question, before I saw the para about being tired of poached-egg-on top. My suggestions are all from the pepper family - pepper jelly, hot pepper sauces like harissa, sriracha, Thai nahm jim, Yemenite zhoug (garlic pepper), or muhammara (walnut pomegranate sweet peppers).

this might be a bit much but in Prince Edward Island i had some very local fish cakes. on the side was something called mustard pickle which could've been the best sauce/jelly/relish i've ever had. it was sweet and salty and acidic and went really well with the fishcake and i think it would go great with a potato cake. I don't have their exact recipe but some googling led me to these...
http://myislandbistrokitchen...
http://www.food.com/recipe...

The Indian in me suggests a mint / cilantro chutney.

I'd also do something with a little zip. The ingredients, celeriac (assuming you're subbing it for the broccoli,) potatoes and cheese all have an affinity for apples, so maybe this apple chutney: http://www.epicurious.com... (I've made it to go with roast pork, and cut back the vinegar a little, as suggested in the comments.)

Wow, these are all great ideas! I kind of want them all...not sure which I'll choose but thanks so much for the suggestions.

And Daniel, I automatically love anything called "mustard pickle" - the recipes you linked to do look a bit too time consuming for me this week, but I am definitely bookmarking them for the future, thanks.

Salsa

Applesauce with a little zest to it.

Smoked Salmon with capers.
Or trout pan fried to crisp up the skin. With ribbons of carrots heated up in the pan. Christened with good olive oil, salt, and lemon juice. Horse reddish sauce optional.

Parsley sauce. https://food52.com/recipes... I'd use a combination of lemon juice and white wine vinegar for this. ;o)

You might like a little finely diced preserved lemon (peel) on top. That tastes bright and refreshing. On the side you could have either arugula or finely chopped parsley dressed with a Dijon, olive oil and sherry vinegar dressing. Not a lot of dressing, and stingy with the oil.

I was thinking more along the lines of steak or prime rib. ;) Or a beautiful salmon fillet.

Bloomfield's caper and lemon dressing would go really well on these - I'd add a fistful of parsley, finely chopped. https://food52.com/recipes...

The lemon, capers and parsley would compliment the broccoli in the potato cakes, while the potato cakes would tone down what is quite a sharp sauce.

Goodness, I just want to go make both of those recipes right now. ;o)