Step #1 Remove any fat from the brisket & cut the meat into 1 inch cubes; set aside.
Step #2 In a 5-quart stockpot, 4 c of beef stock; bring to a boil, mix together the sauerkraut that has been drained, then add onion, & mustard, sliced garlic, the 2 c of thinly sliced cabbage, also 1 c beef stock.
Step #3 Simmer for 15-20 mins, adding stock as needed.
Step #4 Meanwhile, in a small skillet, tomato paste, make a roux; by melting the butter, stirring in the flour & cooking over moderate heat until a light golden brown; add the 1/2 c of beef stock, salt & pepper, paprika.
Step #5 Stir & cook this until thick & creamy.
Step #6 Now stir in the the hot soup, & add the cubed brisket & cook 5 mins more.
Step #7 Serve in bowls.
Step #8 To garnish, cube the bread & bake in a 350°F oven for 8-10 mins.
Step #9 Add the bread cubes & the shredded Swiss cheese to the soup in the bowls.