Stack three 8-inch squares cheesecloth on work surface. Place celery leaves and next 5 ingredients in center of square. Gather up edges of cheesecloth; tie with kitchen string and set packet aside.
Place brisket and short ribs in heavy large wide pot. Add enough water to pot to cover meat. Bring to simmer over medium heat. Skim any impurities that rise to surface. Reduce heat to maintain gentle simmer. Add packet, onion, and 1 tablespoon coarse salt. Cover; simmer until meat is fork-tender, about 2 hours for short ribs and 3 1/4 to 3 1/2 hours for brisket. Transfer meats to 15x10x2-inch glass baking dish. Remove and discard bones from short ribs. Remove and discard packet and most of onion from cooking liquid. DO AHEAD: Can be made 1 day ahead. Cool cooking liquid and meats slightly, then chill separately until cold. Cover each; keep chilled.
Bring meat cooking liquid to boil; add celery root, rutabaga, and potatoes. Reduce heat to medium and cook uncovered until vegetables are tender, about 45 minutes. Drain vegetables, reserving 3 cups cooking liquid. Return vegetables to pot and stir over low heat 1 minute to dry. Mash vegetables with potato masher to coarse puree. Mash in butter. Season to taste with salt and pepper. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. Rewarm over medium-low heat before serving, adding reserved meat cooking liquid by tablespoonfuls as needed to moisten.
Preheat oven to 375°F. Whisk horseradish, mustard, and sugar in small bowl. Brush 3 tablespoons horseradish sauce over meats in dish. Sprinkle with salt and pepper. Roast meats until browned and heated through, 35 to 45 minutes. Transfer meats to work surface. Thinly slice across grain. Place on platter. Drizzle 2 cups reserved cooking liquid around. Serve with vegetable mash and horseradish sauce.

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Nutrition Facts

Per Serving Per 100 g

Amount Per 1 Serving
Calories	843.54 Kcal (3532 kJ)
Calories from fat	526.51 Kcal
		% Daily Value*
Total Fat	58.5g	90%
Cholesterol	236.25mg	79%
Sodium	1206.12mg	50%
Potassium	1061.81mg	23%
Total Carbs	13.41g	4%
Sugars	7.57g	30%
Dietary Fiber	3.39g	14%
Protein	62.93g	126%
Vitamin C	3mg	5%
Iron	6.7mg	37%
Calcium	54.3mg	5%

Amount Per 100 g
Calories	172.92 Kcal (724 kJ)
Calories from fat	107.93 Kcal
		% Daily Value*
Total Fat	11.99g	90%
Cholesterol	48.43mg	79%
Sodium	247.25mg	50%
Potassium	217.67mg	23%
Total Carbs	2.75g	4%
Sugars	1.55g	30%
Dietary Fiber	0.69g	14%
Protein	12.9g	126%
Vitamin C	0.6mg	5%
Iron	1.4mg	37%
Calcium	11.1mg	5%

* Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
Find out how many calories should you eat.

Tastes

salty
savory
bitter
sweet

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