Serves 2
1 tbsp olive oil
1-2 large yellow onion, thinly sliced
1 tbsp rice vinegar
3 tbsp butter, soft
2 tsp miso paste
1 tbsp whole-grain mustard
4 slices sandwich bread
4 oz (1 cup) grated gruyere cheese
In a large skillet over medium-high heat, heat oil. Add onion, stirring often until the onions start to soften and turn brown - about 5-6 minutes.
Reduce heat to low, stirring occasionally, until the onions are deeply browned and sticky (about 20-40 minutes). Stir in vinegar and transfer onions to a small bowl.
In another small bowl, mix 1 tbsp butter with the miso and mustard. Spread on one side of each bread slice and the other with the remaining butter.
Put bread, butter side down, in skillet. Add onions and cheese, put bread, butter side up on top. Repeat for all sandwiches.
Cook until golden brown. Resist the urge to press the sandwich down with the spatula. Flip and again, wait to bread is golden brown.
Transfer to plates and serve - good with pickles on the side (Cornichons/gherkins)