Preheat the oven to 400°F (200°C). Then heat some butter or olive oil in a cast iron skillet over medium heat and saute the onions and garlic until the onions are soft and translucent.
Add the asparagus and season with salt and pepper. Cook the veggies until just tender, about 5 minutes, then remove the pan from the stovetop and transfer the veggies to a large bowl or plate to cool.