Cut the sirloin roast into 1/2-inch by 3-inch strips.
Slice the onion into slices.
Remove the stems and seeds from the bell peppers and slice into 1/2-inch wide strips and set aside until near the end of cooking.
Add the oil to a large pan and heat over medium-high heat until hot.
Add the cut beef strips to the hot oil and cook, turning occasionally until all sides are browned. You may need to do this in 2 batches if your roast was large or your pan is small.
Add the sliced onions, tomatoes, beef broth, soy sauce, sugar, and garlic powder to the pan and stir to combine.
Continue cooking on medium-high heat until the liquid is bubbling.
Reduce the heat to a slow simmer and cover with a lid. Simmer for 1 1/2 to 2 hours, or until the meat is nearly tender.
Add the sliced bell peppers and continue cooking until the beef is tender, about another 30 minutes.
Combine 1 tablespoon of cornstarch with 2 tablespoons of water and add the pan and cook until the liquid is slightly thickened.
Remove from the heat and serve over rice or noodles.
Chuck roast can be used as a substitute.
Any color of bell peppers can be used. Red, yellow, and orange are a bit sweeter than green.
For firmer peppers, add during the last 10 minutes of cooking.
Petite diced tomatoes can be replaced with a 14.5 ounce can of stewed tomatoes. Break them up a bit if you don’t want large pieces.
Condensed beef consommé can be used instead of beef broth. 1 bouillon cube dissolved in a cup of hot water can also be used as a substitute.
To make in a slow cooker or Crockpot; for the best flavor, brown the beef in the oil using a skillet prior to placing into the slow cooker. Add the beef, and everything except peppers to the slow cooker and stir to combine the cornstarch well. Cook on high for 3 to 4 hours, or low for 7 to hours, adding the peppers during the last hour of cooking.
Nutritional data supplied for reference only, I am not a registered dietician. Data based on using 2 pounds of beef.