Combine the rosemary, thyme, shallot, garlic, peppercorns & grapeseed oil in a blender & puree until smooth.
Put the steak in a large resealable bag & pour the marinade over it. Refrigerate overnight.
Light a grill or preheat a grill pan. Pat the steak dry and season with salt & pepper. Grill over moderate heat until medium-rare, about 3 minutes per side. Transfer to cutting board & let rest 10 minutes, leaving the grill on.
In a bowl, toss the peppers with 1 Tbsp of the oil and season with the salt & pepper. Grill until lightly charred.
Grill the lemon halves, cut side down, until charred-about 5 minutes.
In a bowl, toss the peppers with the endive, watercress, mint, celery ribs, celery leaves & remaining olive oil. Squeeze 1 Tbsp of the grilled lemon juice over the salad. Season with salt & pepper and toss. Top with the cheese.
Thinly slice the steak across the grain. Serve with the salad & grilled lemon halves for squeezing over the steak.