I saw a grilled cheese with mustard on a menu years ago, and since then, on the rare occasion that I’ll throw caution to the wind and make or order a big grilled cheese sandwich, I have to have some mustard on the side. Mustard makes almost everything better, I’d argue; one of my friends at work and I bonded over our shared love of the condiment. When an open jar is in my midst, I will put it on whatever is closest, especially if that something is a thick slice of buttery brioche with plenty of melty cheese and peppery mustard greens. Are you drooling? If it’s breakfast time where you are, perhaps consider throwing a fried egg into the mix. —CrepesofWrath
Test Kitchen Notes
I am just now licking my fingers after polishing off this sandwich. I loved the tang of the mustard combined with the sharp, gooey cheddar cheese. I didn't have mustard greens so I used rapini leaves instead. Those added one more layer of bite, which was almost too much, but then things were saved by the sweet crunch of the onions. I will say it would be helpful to chop the greens before cooking, so you don't pull away giant pieces as you eat, and that going spare on the oil would be good. All in all, a great mix. A few notes on the instructions. My go to method for grilled cheese is to grate the cheese instead of slice it, and to grill the sandwiches fully assembled. It saves trying to layer ingredients in over a hot stove. I get the sandwich exterior nice and brown on each side, then pop them into a pre-heated 350 oven on a baking sheet for 5-6 minutes to let the centers completely melt. Worked like a charm here. Highly recommend. —cheesypennies
For the mustard greens
mustard greens, cleaned and roughly chopped
First, cook your mustard greens. Clean and trim the leaves, then heat 3 tablespoons of olive oil in a large pan over medium-high heat. Add in the greens and stir until wilted, about 5 minutes or so. Season with salt and pepper to taste, then remove from the pan and set aside. Wipe out the inside of the pan with a paper towel.
Butter both sides of each slice of bread, then reduce the pan’s heat to medium and add in 4 slices of bread at a time (or however many slices your pan can hold) and toast one side. Flip two of the slices of bread (so that the toasted side is up) and add 1 1/2 tablespoons of mustard, followed by 3 to 4 slices of cheese, 1/4 of the mustard greens, a few slices of red onion, and an additional 3 to 4 slices of cheese. Top these two pieces with the other two slices of bread, placing them toasted side-down onto the cheese. Use a spatula to press down on the sandwiches, then cover the pan lightly and cook for 3 to 4 minutes.
Remove the cover, flip the sandwiches, and cook for another 3 to 4 minutes, pressing down again with a spatula. Remove the sandwiches from the pan when the cheese is gooey and melted, then finish with the remaining sandwich ingredients (you can keep the sandwiches warm in a 250° F oven). Serve warm.