Pat the beef dry and season with salt and pepper. Rub with mustard and sprinkle the walnuts on top (don't worry about being pretty some will fall off while grilling).
Remove the leaves from the thyme sprigs and sprinkle over top of the beef as well. Press gently with fingers to 'seal' the toppings in place.
Reduce the heat on your grill now that everything is nice and hot.
Place the brussels sprouts, shallots, and tomatoes in a grill plate or on a preheated skillet and season with salt and pepper.
Toss with the olive oil and cook until tender on the grill, about 20 - 25 minutes. (If using a slotted grill pan, watch that oil).
Meanwhile, cook the beef, turning as needed for about 20 minutes (depending on thickness) until desired doneness.
Remove everything from the grill and cover with foil. Let the beef rest at least 5 minutes before slicing to serve.
Notes
BGH Specialty Interest Magazine, Eat Well Lose Weight 2014