Try to purchase chicken breasts of similar size so they cook at the same rate. We preferred using 12-ounce chicken breasts in this recipe. Breasts weighing about 1 pound each will require 40 to 50 minutes of roasting. If you can find ground fennel seed, use 2 teaspoons in place of grinding your own. We liked the flavor and texture of grainy mustard, but regular Dijon worked, too. The chicken breasts should fit comfortably in a single layer in the center of the roasting pan with the onion wedges around the perimeter; if your pan is too small, a broiler pan without the slotted top is a good alternative.