Layer russet slices and half the onions in the bottom of a 6-quart slow cooker. Add lamb, carrots, Yukon Gold chunks and remaining onions.
Sprinkle with thyme, salt and pepper, add the bay leaf and pour broth around the sides. Cover without stirring the layers together.
Cook on low for 7 to 8 hours until lamb reaches at least 145°F and is fully fork-tender and the larger potatoes are soft.
Discard the bay leaf and skim excess surface fat. Stir gently from the bottom so the softened russet slices break down into the broth without crushing all the Yukon Gold pieces.
Leave the lid off for 10 to 15 minutes to let the gravy settle slightly, then divide among 6 bowls.