Roast the pork until cooked through and tender, but still nice and juicy about 45 minutes per kilo/20-25 minutes per pound. You can do this on your stovetop over a gentle flame or in a moderate (180C/350F) oven.
About 30-45 minutes before you figure the roast should be done cooking, add the pearl onions to the casserole, seasoning them well with salt and pepper.
Check the roast every 15 minutes or so. Baste the roast with its juices or, if needed, drizzle in a bit of wine or water, just enough to keep things moist. Give the onions a stir and, if you're cooking on the stovetop, turn the roast over as well.
If you'd like to brown the roast a bit more, take the cover off the casserole for the last 15 minutes or so, and raise the oven temp to 200C/400F.