Preheat oven to 450 degrees F (230 degrees C). Lightly grease a baking sheet.

Slice butternut squash cross-wise into 1-inch slices; peel and clean each slice, removing and discarding seeds and stringy pulp from the center. Cut cleaned slices into 1-inch cubes.

Toss squash cubes, onion, and olive oil together in a bowl until coated; transfer to the prepared baking sheet.

Roast in the preheated oven until squash is tender and starting to brown, 25 to 30 minutes.

Toss squash mixture, spinach, dried cranberries, and pecans together in a serving bowl and serve warm.