ground cumin
ground mustard
cayenne pepper
plus 1 tsp. kosher salt, divided, plus more
Freshly ground black pepper
(3 1/2- to 4-lb.) whole chicken
baby Yukon potatoes, quartered
large leeks, white and light green parts sliced into 1/2" rounds
extra-virgin olive oil
(1 stick) unsalted butter, melted
tomato paste
fresh thyme leaves