Step 1Melt 2 Tbsp butter in a bowl (microwave for about 20 seconds) then add the oyster sauce, soy sauce, brown sugar, and sesame oil. Stir to combine, then set the sauce aside.Step 2Bring a large pot of water to a boil. Once it reaches a boil, add the spaghetti and continue to boil until the spaghetti is tender (about 7 minutes). Drain the spaghetti in a colander.Step 3While the spaghetti is cooking, begin the beef and broccoli. Add the remaining tablespoon butter to a large skillet and melt over medium heat. Add the ground beef and continue to cook over medium heat until the beef is cooked through (5-7 minutes). If you're using a higher fat content beef, drain the fat before moving onto the next step (I used 10% fat and there wasn't enough to drain).Step 4While the beef is cooking, mince the garlic. Add the garlic to the skillet with the browned beef and continue to cook over medium heat for 1-2 minutes more, or until the garlic has softened slightly.Step 5Add the frozen broccoli florets to the skillet with the beef and garlic. Continue to sauté over medium heat just until the broccoli is no longer frozen. The broccoli will continue to cook in the next steps, so it does not need to be completely heated through at this point.Step 6Finally, add the cooked and drained pasta and the prepared sauce to the skillet with the beef and broccoli. Continue to cook and stir until everything is combined, coated in sauce, and heated through.Step 7Slice the green onions and sprinkle over top just before serving, along with a pinch of crushed red pepper flakes, if desired.