2 yellow onions, peeled, halved, and sliced
4 TBSP olive oil, divided
generous pinch fine sea or kosher salt
12 oz thickly sliced (about 3/4 inch) portobello mushrooms
1 1/2 TBSP freshly cracked black pepper
1 tsp ground coriander
1/2 tsp smoked paprika
1/4 tsp garlic powder
1/4 tsp smoked salt
1 15-oz can Great Northern beans
1 TBSP balsamic vinegar
4 whole wheat buns, cut sides toasted
Heat a skillet with 2 TBSP of oil on the low side of medium-low heat. Add sliced onions, cover, and cook, stirring and scraping every five minutes or so until deeply browned, about an hour. (Reduce heat as needed if onions threaten to burn.) Meanwhile, heat oven to 400 and lightly oil a roasting pan.
Whisk remaining 2 TBSP oil with all spices, pour over mushrooms in a large mixing bowl, and gently toss to combine (it will coat a little unevenly, but will settle out when you toss them in the oven after the mushrooms release their juices). Transfer mushrooms to pan and sprinkle on any spices that fell to the bottom of the bowl. Roast 40 minutes, tossing well after 20 minutes.
Meanwhile, rinse beans well and set aside in a sieve to let drain thoroughly. Add to a food processor when the onions are almost done.
When the onions are ready, add balsamic vinegar and use it to deglaze the pan one last time before removing from heat. Add onions to processor and process until smooth. Set aside to let cool a bit while you toast the buns.
To serve, spread dijon mustard on bottom bun, add one quarter of the mushrooms, then top with sauerkraut. Spread bean mixture on top bun, sandwich, and serve immediately.