These are the best tasting sirloin tips. Or at least, they are the best I've ever made. The secrets to the beefy flavor are using beef fat (saved from making stock) to brown the beef and using beef demi-glace paste in the liquid the beef is braised in.
Braising the beef in the oven means you don't need to watch it while it simmers.
Cut the beef into 1 inch (or slightly smaller) cubes, discarding any fat or gristle. Most of the "silver skin" can remain as it will melt in the long cooking.
Toss the beef in the black pepper.
Using a large Dutch oven or similar oven-safe pan with cover, melt 2 Tbl of the beef fat on medium-high heat. Brown the beef in small batches, adding fat as necessary. Adjust the heat to prevent the pan drippings from burning. Remove the last of the beef from the pan.
Discard any excess fat leaving enough in the pan to lightly saute the onions and garlic. Cook the onions and garlic for 2 to 3 minutes, stirring to coat them with the pan drippings.
Stir together the wine, soy sauce, water and demi-glace paste until the paste is dissolved.
Add the browned beef cubes and the liquids to the onions and garlic and bring to a simmer.
Cover and simmer in the oven for 1 1/2 hours or until the meat is fork tender.
Continue to simmer on top of the stove and slowly add the dissolved cornstarch, stirring to prevent lumps. This should quickly thicken the liquid to a rich gravy. Simmer another minute, if the gravy has not thickened to your liking, add more dissolved cornstarch but be careful! Too much cornstarch will turn the gravy into a sticky goo.