Heat the vegetable oil over medium-high heat in a high sided saute pan or dutch oven. Then add the cubed sirloin beef in one layer to sear, cooking about one minute per side. Season them with salt and pepper as they're cooking. Do not overcrowd the pan, cook the cubes in batches or else they'll steam and turn grey instead of sear and turn brown.
After all sides have seared, remove the cubed beef onto a plate and repeat the searing process with the next batch until all the beef is seared and put onto a plate.
Melt the 4 tablespoons of butter in the same pan over medium heat, then add the diced yellow onions and sliced mushrooms. Saute the vegetables for about 6-8 minutes, stirring with a heat-safe wooden spoon or spatula occasionally so they don't burn, or until the onions and mushrooms are soft and have gained a bit of color.
Add the minced garlic cloves and saute them for 1 minute until fragrant. Add the 2 tablespoons of flour and stir constantly for another minute to cook out the raw flour taste.
Whisk in one cup of beef broth, whisking constantly to incorporate the roux. It will get gloppy and thick and seize up, that's perfectly normal! It will loosen up as the broth is added. Once the first cup is added, pour in the rest of the beef broth, whisking quickly to ensure there are no lumps.
Add the beef cubes back in, along with any acculuated juices on the plate, add in the brandy, Dijon mustard, and Worcestershire sauce, and let simmer for about 15-20 minutes, or until the stroganoff sauce has reduced and thickened somewhat.
Reduce the heat to low and add the sour cream to the stroganoff. Stir it in well. Taste the sauce and season with salt and pepper to taste, serve hot over desired sides.
Keep in an air tight container in the fridge for up to 3-4 days.