Slice the short, round bulbous end off the squash and reserve for another purpose. Trim the top from the neck (long side), then peel the skin off the squash neck using a vegetable peeler or very sharp knife.
Using a handheld julienne peeler, julienne the neck into long squash noodles. Alternatively, create the noodles with a spiralizer.
Toss the noodles with the avocado or olive oil, then spread between two sheet pans in a single layer.
Transfer to the oven, and roast the squash until tender and al dente, about 7 minutes, stirring the noodles halfway through.
While the noodles are roasting, heat a 12” skillet over medium heat. Add the butter and heat until it melts. Add the sage and saute, stirring continuously, until the sage turns a deep green and starts to crisp, about 3 minutes. Add the garlic to the sage butter and saute, stirring continuously, until the garlic is fragrant and starting to brown around the edges, 1-2 minutes.
Remove the butternut squash noodles from the over and season with salt and pepper. Add the roasted squash noodles to the pan and toss to coat in the sage butter. Saute the noodles in the sage butter, tossing with tongs, for an additional 1 minute. Remove from heat and serve immediately.