PWMTrav

If you're planning one (expensive), high end meal and are reasonably adventurous eaters, I recommend Hugo's. They're on OpenTable, but calling is better. They do tasting menus only - it's been that way for a long time, before they were trendy, even. Used to be one blind tasting menu each night, but now they give you a choice of 2-3, but you could go with a blind tasting if you want. Last I was there, I think it was $90. Add $60 if you want to do wine pairings. If you like wine, it's definitely not an afterthought here, and it's all paired per course. So that's $300 for two before tax, tip, and any other drinks you order. Still, if I'm going all out around here, I can't recommend it highly enough.

As far as lobster risotto, it's not regularly on the menu of any place I've been recently. I see it, but usually as a special. I don't want to promise you it'll be somewhere and then have it not show up. The truth is, Portland restaurants rotate menus frequently - some do it daily. The Salt Exchange usually had lobster risotto, but they closed and I'm still not sure why, it was a good restaurant.

I forgot one place, no reservations but you won't need one. Fishermen's Grill. It's not walking distance, but close enough to cab. It's more of a seafood shack, and the lobster rolls here are traditional style and VERY good. Also recommend the chowder or anything else on the menu, really.
http://www.fishermensgrill.com

A lot of the places I like don't take reservations, which is kind of a pain since I have a 2 year old! I'd say it's still worth trying to eat at some of them - specifically Duckfat, Eventide, Pai Men Miyake and Boda, as well as Hugo's if that's your thing. Since it's just two of you, go a little on the early side and the wait shouldn't be bad. Those are the 5 places I'd take friends coming to visit me if it were their first time in Portland.