Only apply the miso butter to the cheese-facing inner side of the bread—not the side that hits the pan as the direct heat will burn the miso.
- Melt 1 tablespoon of butter in a large nonstick skillet set over medium-high heat. Add the oil, onion, and salt and cook, stirring often, until the onion is soft and browned in spots, 8 to 10 minutes. Stir in the vinegar and turn off the heat
- In a small bowl mix the miso butter with the mustard, then divide it over one side of each bread slice, spreading it out evenly. Turn the bread slices over, and spread the other side with the remaining 2 tablespoons of butter.
- Place 2 of the bread slices, plain-butter-side down, in the skillet. Divide the cheese and onion evenly over the slices and cover with another slice of bread, plain-butter-side up (if the miso-buttered side is directly exposed to heat, it will burn).
- Cook the sandwiches over medium-low heat until the bread is golden-brown, 4 to 5 minutes (do not press the sandwich down with a spatula-just let it be). Flip the sandwiches over and cook on the other side until the bread is golden brown and the cheese is melted, 3 to 4 minutes longer. Transfer to plates and serve with pickles. Lots of pickles.
Adapted from Umami Bomb: 75 Vegetarian Recipes That Explode With Flavor by Raquel Pelzel (Workman Publishing). Copyright © 2019.
Raquel Pelzel is the Editorial Director for Clarkson Potter Publishing, a division of Penguin Random House. She has co-authored more than 20 cookbooks and was formerly an editor at Cook’s Illustrated and the food editor of Tasting Table. Her next cookbook, Sheet Pan Suppers – Meatless, comes out in Fall 2017.