Roasted Fennel (optional):
To prepare brine, combine water, salt and sugar in 20-quart food-safe container; stir to dissolve salt and sugar. Cool water 1 hour; add beef roast. Cover and refrigerate overnight or up to 24 hours.
Preheat oven to 350°F. Remove roast from brine; discard brine. Pat dry roast with paper towel. Combine Seasoning ingredients in small bowl; press evenly onto all surfaces of roast. Place roast, fat-side up, in shallow roasting pan. Insert ovenproof meat thermometer so tip is centered in thickest part of beef, not resting in fat or touching bone. Do not add water or cover. Roast in 350°F oven 2-1/2 to 3 hours for medium rare; 3 to 3-1/2 hours for medium doneness.
Meanwhile, prepare Roasted Fennel, if desired. Trim off and save fronds from fennel bulbs; cut off stems to within 1 inch of bulbs and discard stems. Cut each bulb lengthwise into quarters. Place on rimmed baking sheet. Drizzle with olive oil; toss gently to coat, keeping fennel pieces intact. Season with kosher salt; arrange cut-sides down. About 1-1/2 hours before roast is done, place fennel in lower third of oven. Roast for 1-3/4 to 2 hours or until tender and lightly browned, turning and rearranging once. Just before serving, toss fennel with 1/4 cup chopped fennel fronds.
Remove roast when meat thermometer registers 135°F for medium rare; 150°F for medium. Transfer roast to carving board; tent loosely with aluminum foil. Let stand 15 to 20 minutes. (Temperature will continue to rise about 10°F to 15°F to reach 145°F for medium rare; 160°F for medium.)
Carve roast into slices; season with salt, as desired. Serve with Horseradish Cream Sauce and Roasted Fennel, if desired.
Nutrition information per serving: 562 Calories; 405 kcal Calories from fat; 45 g Total Fat; 19 g Saturated Fat; 18 g Monounsaturated Fat; 135 mg Cholesterol; 1696 mg Sodium; 4 g Total Carbohydrate; 0.4 g Dietary Fiber; 34 g Protein; 3.5 mg Iron; 5 mg Niacin; 0.4 mg Vitamin B6; 3.6 mcg Vitamin B12; 7.8 mg Zinc; 32 mcg Selenium; 124 mg Choline;