Step 1: Chop ingredients
2 large onions, coarsely chopped
~2 lbs sweet potatoes, sliced thinly
handful garlic, coarsely chopped
1-2 stems rosemary, chopped
salt & pepper to taste
heavy drizzle of olive oil
Combine ingredients in a 9x13" pan.
Step 2: Mix
Check the seasoning by giving one of your fingers a lick on the way to the sink. It should be reasonably salty, because the sweet potatoes suck up salt as they cook.
Cover the pan in foil; at this point you can set it aside in a cool place while you prepare the rest of your Thanksgiving dinner. You can even do it a couple of days in advance, though I'd recommend adding a bit of lemon juice to the mix to help prevent oxidation.
Step 3: Bake
Step 4: Serve
Leftovers save extremely well, and are particularly good additions to omelettes or scrambled eggs.