Prep the green beans by blanching them in boiling water for 3 minutes, then transfer to a bowl of ice water. Once cool, transfer to a towel to dry and set aside.
Make the Tomatillo salsa and chill until ready to use.
Make the filling. Heat a grill to medium heat and grill the blanched green beans and the whole ears of corn until charred on all sides.
Chop the green beans into 1” pieces and add them to a large bowl with the onion and tomatoes. Slice the kernels off the corn and add those to the bowl as well. Add the olive oil, lime juice, cilantro, pinches of salt and pepper, and a few spoonfuls of the tomatillo salsa and toss. Season to taste.
Grill or warm the tortillas. Assemble the tacos with the corn taco filling, feta cheese, and serrano slices, if desired. Top with salsa and serve the extra on the side.