Scatter the onions in the bottom of the slow-cooker. Put on the lid and set to high.
Season roast generously with salt and pepper.
Heat oil in a cast-iron skillet over high-heat. Place the roast in the pan and leave it in place until it releases in order for it to form a golden brown crust. Rotate to an unbrowned area and repeat. Do this until the entire roast is browned. Place on top of the sweet potatoes.
Cover with sweet potatoes and sliced peaches. Cook on high 4-5 hours or low 7-9 hours.