Adjust oven rack 6-8 inches from the broiler. Line a baking sheet with aluminum foil and broil chiles until they begin to blister and char, about 4 minutes. Flip and broil the second side. Allow chiles to cool (about 10 minutes) by placing them in a sealed plastic bag. Peel off any loose skin.
Heat olive oil in a medium skillet over medium heat. Add onions and cook until translucent then add shredded beef and ground cumin and cook until browned. Drain. Transfer beef mixture to a large bowl and allow to cool. Add cheese and garlic salt to bowl and stir to combine.
Move oven rack to middle position and preheat oven to 350 degrees.
Make a slit lengthwise along each chile and remove seeds. Stuff each chile with beef and cheese mixture until full. Place stuffed chiles into a baking dish and into the oven. Bake for 25-30 minutes or until heated through. Serve with sauce. Garnish with cilantro if desired.