Step 1

Season the roast evenly with the salt and pepper. Refrigerate overnight, or for up to 48 hours.

Step 2

One hour before cooking, remove the tenderloin from the refrigerator. Fold the tail of the tenderloin over and truss it to even out the thickness on the thin end of the roast. Cut the roast in half, truss the two pieces together, then skewer on the rotisserie spit, running the spit between the pieces of tenderloin and securing them with the spit forks. Whisk the mustard, garlic and thyme, then brush all over the trussed tenderloin. Let the beef rest at room temperature until it is time to grill.

Step 3

Set up the grill for indirect high heat (450°F or higher). On my Weber Summit, I remove the grill grates and preheat the grill with all burners on high for 15 minutes. Then I turn off all but the outer burners - burners 1 and 6 - and turn on the rotisserie burner to high.

Step 4

Put the spit on the grill, start the motor spinning, and make sure the drip pan is centered beneath the tenderloin. Close the lid and cook with the lid closed as much as possible. (If you have an infrared rotisserie burner, turn it off after the beef is browning well, about 30 minutes, and let the burners in the main body of the grill finish cooking the beef.) The tenderloin is done when it reaches 120°F in its thickest part for medium rare, about 45 minutes. (Cook to 115°F for rare, 130 ° F for medium.)

Step 5

Remove the tenderloin from the rotisserie spit and immediately remove the trussing twine. (Be careful - the spit and forks are blazing hot.) Let the tenderloin rest for 15 minutes, then slice and serve.