In a large sauté pan over medium heat, heat enough vegetable oil to fill about an inch up the pan. Heat the oil to 350°F (180ºC).
Add the milk to a medium mixing bowl and whisk together the flour, salt and pepper in another medium bowl.
Dip a handful of the onions into the milk then toss them in the flour mixture to coat. Add to the hot oil and fry until golden brown all over. Remove to a paper towel-lined plate to drain.
For the chipotle mayo:
In a small bowl whisk together the mayonnaise and chipotle powder. Season to taste with salt and pepper.
Cover and refrigerate until ready to use.
For the burger:
Divide the meat into 12 equal portions and gently form them into patties. Season with salt and pepper on both sides.
Heat a medium cast iron pan over medium-high heat and add several patties to the pan. Be careful not to over-crowd the pan.
Cook until browned on the outside on both sides and cooked until just pink on the inside, about 4 minutes each side. Remove to a plate and continue to cook the remaining patties.
Spread some of the chipotle mayo on the lower half of each slider bun then add a patty and top with avocado slices, roasted corn, jalapeño slices and crispy onion strings. Serve immediately.