Which onion variety is commonly used for fajita mixes?

While red onions are enjoyed for their sweet taste and vibrant color, they are not the most common choice for fajita mixes. Yellow onions are often preferred for this purpose as they have a balance of sweetness and sharpness, which caramelizes well when grilled. The flavor of yellow onions intensifies when cooked, complementing the other ingredients typically found in fajitas.

White onions have a sharper taste and are often used in salsas or garnish but aren't the primary choice for cooking fajitas. Green onions, while they can be used for garnishing or as a fresh component in fajitas, don't form the hearty base of flavor expected in the actual dish.

Therefore, yellow onions are the most commonly used variety in fajita mixes due to their ability to blend flavors and add depth when grilled alongside peppers and meats.