For the Onions:

1. Place the sliced onion in a shallow baking dish or bowl and pour the buttermilk over top. Soak the onions for at least an hour.
2. In another dish, combine the flour, salt, and pepper, whisking well. Set aside.
3. Heat the canola oil up in a cast-iron skillet until a thermometer reads 375°F.
4. Remove the onions from the buttermilk, allowing the excess to drip off, and then dredge through the flour mixture until evenly coated. Shake off the excess flour.
5. In batches, add the breaded onions to the hot oil and cook, stirring them around, for 30 seconds or until golden brown and crispy. Remove with a slotted spoon and place on paper towels to drain.
6. Sprinkle with a little salt while they are hot out of the oil.
7. The onions can be made up to a day ahead of time. Allow them to cool completely on the paper towels so there is no excess grease and store them well wrapped at room temperature.

For the BBQ Spice Mix:

Mix all the spices together and then store in a small resealable container or jar. It will keep for months in a cool, dry place.

For the BBQ Mayo:

Mix all the ingredients together in a bowl and adjust the seasoning as needed. You can make this ahead and store it, covered, in the fridge.

For the Burgers:

1. Preheat the grill, griddle, or a large cast-iron skillet over medium-high heat.
2. In a large bowl, mix together the beef, BBQ spices, BBQ sauce, salt, and pepper. Gently mix to combine the ingredients without overworking the meat too much (this will give you a tough burger).
3. Form the mixture into four patties.
4. If using a skillet, add a drizzle of canola oil to the pan. Add the burger patties to the grill/griddle/hot pan and cook for 4 minutes per side. During the last minute of cooking, add the slices of cheese to each burger.
5. While the burgers are cooking, spread the burger buns with the BBQ mayo.
6. Place the cheeseburgers on the buns and top with lettuce, pickles, and the crispy onions. Serve immediately.