Preheat oven to 350º F.
Fry up the bacon in a large skillet and drain on a paper towel, reserving 2 tablespoons of the bacon grease.
Thinly slice the Brussels sprouts using the slicing disc of a food processor. You can also use a mandoline or slice them by hand, but it will take much longer, and the slices are unlikely to be as thin and evenly sliced as with the food processor. Dump into a large bowl and add the cranberries and cooked quinoa.
Heat up 2 tablespoons of the bacon grease, and add the broth, whipping cream, mustard, maple syrup, celery salt, kosher salt and pepper. Whisk and heat just until starting to simmer. Add in half of the shredded Brussels sprouts and carefully turn to coat using tongs. Sauté for 5 minutes and return to the bowl with the remaining Brussels sprouts. Toss thoroughly and scrape the mixture to a shallow baking pan (I used a 9" X 11" pan). Spread in an even layer.
Stir the melted butter and maple syrup together in a small bowl. Place the bread crumbs, pine nuts, and salt in a small bowl and toss with the melted butter and maple syrup until the crumbs are well-coated . Spread evenly over the Brussels sprouts.
Bake the gratin until the topping is golden brown. This should take about 45 minutes at sea level and 1 hour at high altitude (6000-7000 feet). Rotate the pan half-way through to promote an even browning.
I've made this with and without bacon, and my family prefers it with. But please feel free to substitute vegan "bacon pieces" or eliminate altogether. Use 2 tablespoons butter in place of the bacon grease for mixing with the broth, whipping cream, maple syrup, etc.