Bring chicken stock to a simmer. Whisk in polenta.

Cook for 30 minutes or until tender, whisking frequently. Add butter.

Line a deep-rimmed baking sheet with wax paper. Pour polenta onto tray and spread evenly to 1" thick. Allow to cool at room temperature; transfer to refrigerator and chill for a minimum of two hours.

Remove polenta from the refrigerator when completely chilled and firm. Cut circles into the polenta using a 2" cookie cutter. Scoop out the centers of each polenta circle with a 1/2 teaspoon measuring spoon, taking care not to make a hole in the bottom of the circle. Polenta cups may be prepared up to 2 days in advance at this point.

Preheat oven to 350 degrees F.

Season beef with salt and pepper to taste.

Heat 2 tablespoons oil in Dutch oven over medium-high heat.

Add meat to pot, searing on all sides until dark brown. Remove meat from pot.

Add shallots and garlic to the Dutch oven. If necessary add remaining oil. Sweat for 5 minutes.

Add tomato paste and cook until paste becomes fragrant and dark red, approximately 3 minutes.

Reduce heat to low and add flour.

Whisk in beef stock, breaking up any lumps. Add wine and fish sauce. Simmer over low heat until the sauce is thick enough to coat the back of a spoon.

Add the reserved beef to the sauce and bring to a boil. Cover and place in oven. Cook until tender, about 1 1/2 hours. (Sauce should continue to be at a light simmer while in the oven. If the sauce stops simmering bring it back to a simmer on the stove top and return to the oven.)

Remove the beef from the sauce, cover and reserve.

Strain sauce through a fine mesh strainer and reserve.

Shred beef while warm, discarding any pieces of fat. Toss shredded beef with some of the reserved sauce to coat. Reserve warm.

Warm polenta cups in a 250 degrees F and fill with shredded beef.