Chop the fat you reserved from trimming the beef into small cubes. Cook in the pot over medium heat until it's released about three tablespoons of fat. Discard the solids. If you don't have enough beef fat, you can substitute vegetable oil.
Increase the heat to medium-high and brown the cubed beef on all sides. Remove with a slotted spoon and set aside. You may need to do this in three batches to prevent overcrowding.
Add the diced peppers and onions and saute until the onions are translucent and beginning to brown. About ten minutes.
Add the beef stock, potatoes, tomatoes, seasonings, garlic, and meat. Stir well, bring to a simmer, then reduce heat to a low simmer. Cover and cook for one hour until the meat is tender. Taste for salt and adjust.
Remove 1 cup of liquid and stir in the flour until smooth. Add back into the pot and thoroughly mix in. Cook uncovered until thickened. About 10 minutes.