Step 1

Place chicken on a large cutting board, breast-side down. Using kitchen shears, cut along each side of backbone and remove (reserve for stock, see cook's note). Flip over; press firmly with your palm in center of breasts to flatten.

Step 2

Whisk together mustard, mayonnaise, rosemary, and lemon juice. Season chicken generously all over with salt and pepper; slather both sides with mustard mixture. Cover loosely and let stand while you prepare the grill, or refrigerate in a resealable bag up to 1 day (return to room temperature 1 hour before grilling).

Step 3

Heat grill to 350°F for direct and indirect cooking. Oil grates over indirect zone, then place chicken, skin-side up, on it, with legs facing direct zone (this helps keep the breast from overcooking before the legs are done).

Step 4

Cover and cook, maintaining a temperature of about 350°, until skin begins to brown and a thermometer inserted in thickest part of breast reads 150°, 45 to 50 minutes.

Step 5

Oil grates over direct-heat zone. Using tongs, carefully flip chicken skin-side down over direct heat. Cook, moving occasionally to avoid flare-ups, until skin is charred in places and chicken is cooked through (thermometer should read 160°), 5 to 10 minutes more. Transfer to a tray; tent with foil. Let stand at least 15 minutes before carving.