Combine marinade mixture for flat steak. Place steak in a resealable plastic bag and pour marinade over it. Seal bag and place into refrigerator for 2 hours.
Preheat grill for medium-high heat.
Add 1 tablespoon of olive oil to the bottom of a 12" cast iron skillet. Working from outside in, lay down the first sweet potato slice and place the second halfway off the next making a circular pattern leading to the center of the pan. Place onto the grill and cook for 15-20 minutes until the sweet potatoes become tender.
Brush corn, onion slices, and jalapeno peppers with olive oil. Place onto grill. Cook corn and peppers, turning often until they have some light charring. Remove from grill and place onto a large cutting board. Grill onions 3 minutes per side, or until they become slightly tender and golden in color. Take off of heat and add with other vegetables.
Remove steak from marinade bag and place onto grill. Cook 4-7 minutes per side, depending on thickness. Once cooked remove and place onto a cutting board.
By now the sweet potatoes should be tender. Add drained pinto or black beans on top of potatoes. Slice jalapenos, reserving half of one for the sauce. Cut away kernels from ear of corn. Chop onions. Place vegetables on top of beans.
Slice meat and place onto vegetables. Top all of it with crumbled queso fresco
Cook for an additional 5-7 minutes or until everything has warmed through and cheese has melted.
Remove seeds from the reserved half of jalapeno. Chop and add to blender with sour cream, lime juice, and salt. Blend mixture until smooth.
Take sweet potato nachos off of grill and place onto a heat resistant surface. It will be screaming hot at this point so let it stand for about 5 minutes. Drizzle with sauce, garnish with cilantro leaves, and serve.