Put the thighs, potatoes and onions in a large bowl. Whisk the butter, honey and mustard together and pour half over the chicken. Toss in some thyme and mix.
Place the potatoes and onions in a tajine. Put the chicken on top and cover. Roast for 30 minutes in a 400 degree oven. Remove from the oven and baste with remaining sauce. Cook for another 15 minutes and baste again, returning to the oven for a final 15 minutes.