2. Skewer the onion slices using two parallel skewers to hold all the layers together. Make sure one skewer pierces the center section so that it doesn't fall out. Rub all slices with olive oil and salt to taste. Set aside.
1. Remove peach seed, remove seed and peel from the mango. Place mango and peach flesh along with remaining onion pieces into a large food processor or blender.
3. Trim the ends off of 1/2 a bunch of cilantro, then add cilantro to the food processor along with the vinegar, salt, and pepper.
4. Blend on high until the sauce is bright green and resembles a pesto or chimichurri style sauce.
2. Grill steak and onions for 7 minutes, then flip. Continue to cook the onion another 7 minutes - 10 minutes or until they are tender and golden brown.
3. Continue to cook the steak until the internal temperature reaches 110 degrees, then remove the steak to a clean plate. Turn that section of the grill to medium high and allow it 5 minutes to heat.
4. Sear all sides of the steak over direct medium high heat for about 60 seconds per side. Check for an internal temperature of 125 degrees and allow it to rest 5 minutes before serving.