Spray a large skillet with cooking spray and heat over medium-low heat. Cook and stir onion in the hot skillet until translucent, about 5 minutes. Add kielbasa; fry until sausage is lightly browned, stirring occasionally, about 5 minutes more.
Stir broccoli and potatoes into sausage mixture; season with salt and pepper.
Cook, without stirring, until broccoli begins to soften, about 15 minutes. Stir mixture and continue to cook until vegetables are completely tender, 10 to 15 minutes more.
1/2 onion, chopped
1 (16 ounce) package kielbasa sausage, sliced
1/2 head broccoli, cut into florets
3 potatoes, peeled and sliced
salt and ground black pepper to taste