In a bowl, combine all the ingredients. Refrigerate.
Preheat the grill, setting the burners on high. Oil the grate.
With your hands or a hamburger press, shape the meat into four patties.
Grill the patties for 4 to 5 minutes on each side or until cooked through. Season with salt and pepper.
Toast the buns. Place the patties on the bottom buns and spread with chipotle mayonnaise. Garnish with fried onion rings. Cover with the top buns.
You can find canned chipotle peppers in adobo sauce in the Mexican food section of most grocery stores. Once the pepper can is open, store them in the refrigerator or freeze in small portions. For a spicier mayonnaise, double the amount of peppers.