Slice the top sirloin beef into thin strips, or cut it further if necessary.
In a bowl, mix together 30 g of cheddar cheese, about half a teaspoon of cornstarch, and 30 g of evaporated milk to create a cheese sauce mixture. Set this aside.
Dice the yellow onion and, if you like, chop some pickled jalapeños.
Heat a pan over medium heat and add a little oil. Optionally, toast the inner side of the baguette for about two minutes.
Add a bit more oil to the pan, then toss in the diced onion with a pinch of salt. Sauté for approximately three minutes, stirring occasionally, until the onion is soft, translucent, and slightly caramelized.
Push the onions to one side of the pan, add a little more oil, and then add the beef strips. Season with a generous pinch of salt, freshly ground black pepper, and garlic powder. Cook for 45 to 60 seconds on each side, adding jalapeños if using, and stir everything together.
Turn off the heat and pour the prepared cheese sauce into the pan, stirring for about thirty seconds until the sauce is creamy and gooey.
Remove the pan from the stove and place the cooked beef and cheese sauce onto the prepared baguette.
Serve the cheese steak sandwich immediately.
