To prepare the corn salad: Use a sharp chef's knife to slice the kernels off all four sides of the corn cobs. Transfer the kernels to a medium-sized mixing bowl and add the chopped cilantro, radishes, lime/zest and juice, jalapeno, olive oil, and sea salt. Mix well. Stir in 1/4 cup crumbled feta, taste, and add a little more if you'd like. Set the bowl aside to marinate while you prepare the beans.
To prepare the black beans: Warm the olive oil in a large saucepan over medium heat. Add the onions and a sprinkle of salt. Cook, stirring occasionally, until the onions have softened and are turning translucent, about 5 to 8 minutes. Add the cumin and cook for about 30 seconds while stirring. Pour in the beans and 1/3 cup water. Stir, cover and reduce heat to simmer. Cook for 5 minutes, then remove the lid and use the back of a fork to mash up about at least half of the beans. Remove from heat, season generously with salt and pepper, to taste, and cover until you're ready to serve.
To warm the tortillas: Heat a cast iron or non-stick skillet over medium heat and warm each tortilla individually, flipping occasionally. Alternatively, you can warm them directly over a low flame on a gas range. Fold a tea towel over the warmed tortillas to keep them warm.
Put it all together: Spread black beans down the middle of each tortilla. Top the beans with corn salad. Place a slice of avocado to the side of the beans and serve with optional pickled jalapenos, salsa, or hot sauce on the side.