To prepare the remoulade, combine the heavy cream, lemon juice, and mustard in a large mixing bowl, and season with salt and white pepper.
Immediately add the julienned celery root to the dressing to prevent the celery from darkening, and toss well. Keep refrigerated and drain slightly before serving, if necessary.
Preheat the oven to 400°F.
Toss the potatoes with the olive oil, spread in a roasting pan, and roast for 45 to 50 minutes, tossing occasionally, until cooked through and golden brown.
After the potatoes have roasted for about 15 minutes, rub or brush the tenderloin with olive oil and season with salt.
Rub in all sides with the crushed peppercorns, pressing gently so they adhere. Place the tenderloin in a roasting pan and roast in the oven for about 30 to 35 minutes for medium-rare, about 5 minutes longer for medium, or to the desired doneness.
Remove from the oven, cover loosely with aluminum foil, and let rest for 10 minutes.
Just before the potatoes have finished roasting, melt the butter in a small sauté pan. Add the garlic and sauté over medium-low heat for 2 or 3 minutes, until it begins to soften. Set aside.
Transfer the potatoes to a large serving bowl and drizzle the garlic and butter over them.
Season generously with salt and pepper, and toss to combine. Keep warm until ready to serve.
Slice the tenderloin and serve with the fries and remoulade.