Preheat oven to 400 degrees.
Begin by preparing the butternut squash to roast. Cut off ends of butternut squash then carefully slice off exterior skin. Slice the butternut squash into 8-12 ½-inch thick rounds. Scoop seeds from the slices around the bulb.
On a parchment-lined sheet pan, drizzle with olive oil then add squash slices in a single layer. Season with salt and fresh-ground pepper then flip squash, and do the same to the other side. Roast on the middle rack of the oven for 15 minutes.
After 15 minutes, flip the squash pieces and add sage leaves across the sheet pan and drizzle leaves with more olive oil. Roast 12-15 more minutes until squash is golden and tender. Set aside.
While squash is roasting, warm a large saute pan, (the same one you will cook your burgers with), over medium heat. Add olive oil followed by your julienned onions. Sprinkle with salt and pepper and cook on medium low for 15-20 minutes, stirring occasionally, until browned and caramelized. Set onions aside to cool and wipe down pan to sear burgers.
In a medium bowl, add ground bison, whole grain mustard, coconut aminos, salt and pepper. Mix burger mixture well then form into 8 uniform patties. In your original sauteed onion pan, heat pan over medium high heat and add another drizzle of oil. Sear burgers in batches, 3-4 minutes each side, until cooked through. Add sliced white cheddar to tops, then add ½ cup water to bottom of cooking pan then cover, the steam will melt cheese and finish cooking burgers.
Transfer each burger patty atop a butternut squash round then finish with sauteed onions and crispy sage. You can either serve open-face or add another butternut squash slice atop, burger-style.