Open-faced Caramelized Onion and Fontina Grilled Cheese Sandwich

It’s almost summer in Boston, but sometimes it doesn’t quite feel like it. Luckily, we’re seeing the seasons change with fresh summer produce turning up in the grocery stores, and the farmer’s markets have opened across the city. Sometimes it’s warm enough to grab a drink outside after work. But as I’m sitting here writing, it’s barely over 60 degrees outside, and I need to wear long sleeves to stay warm with the windows open.

About a week or so ago, it was a chilly, rainy day with the temperature down into the 50s. I’ve started a more ambitious running routine since my semester ended in May, and my good friend Jodi introduced me to running on the Esplanade along the Charles River. I do either a 4 or 5 mile loop most days of the week, and I’d been doing such a good job sticking to my routine that I didn’t want to let the rain and the chill be what broke my good streak. So I wore a fleece pullover and hit the road, got wet, but wound up back home after an otherwise normal run. I spent the rest of the day feeling like the damp had seeped into my bones and couldn’t get quite warm enough no matter what I did.

And so this sandwich was born: Standing by the stove slow-cooking some onions for about an hour and roasting tomatoes for soup would be the only thing to break the chill.

This sandwich is incredibly simple in concept: smear a thick layer of mustard on a hefty slice of fresh bread, top with a few slices of a melt-y cheese like fontina, heap on the onions, and then add more cheese just for fun. Toast it all in an oven until the bread crisps up and turns brown, and the cheese melts and oozes into the pockets of caramelized onions. Pile the fixins on so high that you couldn’t possibly add a second slice of toast and still get a clean bite off. Instead, eat it open-faced style, either with your hands or a knife and fork.

Happily, the forecast for the week ahead is predicting a few 90-degree days, which will be such a welcome change. And it has me thinking this sandwich can be easily translated to an outdoor meal (or appetizer) if you’re grilling. Since making this sandwich for the first time, I’ve made a few batches of caramelized onions to keep in the fridge and use as a sandwich topper, and any extra onions you have would be great on top of burgers or hot dogs. You could also toast the bread on the grill first: lightly coat the slices with olive oil and place them face down over the grates. Let them get warm and toasty with grill marks, then rub a clove of garlic over the toasted side before layering on the mustard, cheese, and onions.

Open-faced Caramelized Onion and Fontina Grilled Cheese Sandwiches

Ingredients
4 medium yellow onions, thinly sliced
2-3 Tablespoons olive oil
4 slices of a large french bread loaf, or baguette cut along diagonally
2 Tablespoons whole grain mustard
about a 3-inch hunk of fontina cheese

Steps
1. Add the onions and the olive oil to a large skillet or pot. Over low heat, gently cook the onions, stirring occasionally, until they caramelize and turn a deep golden brown but don’t burn (at least 30 minutes, probably more like 45).

2. Heat the oven to 300-degrees. Smear the slices of bread with the mustard in an even layer. Top with two slices of fontina cheese, then layer the caramelized onions on top. Sprinkle with more grated fontina. Toast in the oven for about 10 minutes, until the cheese is thoroughly melted and the bread crisps up a bit. Serve warm.