In a large bowl, mix together the thinly sliced red onions, thinly sliced jalapeños, ⅓ cup lime juice, and 3 tablespoons of the extra virgin olive oil. Season with kosher salt and black pepper and let the mixture marinate for at least 20 minutes, or up to 1 hour.
Position an ear of corn upright in a large bowl. Use a knife in a downward direction to shave the corn kernels, allowing the kernels to drop into the bowl.
Heat a large cast iron skillet over medium-high heat with the remaining 1 ½ tablespoons of the extra virgin olive oil. Add HALF of the corn kernels. Cook the corn for 3-5 minutes until charred in spots, tossing just once or twice to get a nice char on them.
Add the charred corn to the bowl with the marinated onions and jalapeños. Add the raw corn to the bowl, along with the sliced cherry tomatoes and drained beans. Season to taste with salt and pepper and the juice from the extra lime, toss to combine and taste again for seasonings.
Add the basil and toss to combine. If serving the salad immediately, dice the avocado and mix gently to combine. If keeping some of the salad as leftovers, add the avocado only to the portion you plan to eat now.
