For the radish salsa: Thinly slice and quarter the radishes. Thinly slice the green onions. Seed and dice the jalapeño pepper (taking the necessary precautions to keep the juice away from the eyes). Use 1/2 pepper for a mild salsa, and more for a spicier variation. Chop the cilantro. Mix the cilantro with radishes, green onions, jalapeño pepper, lime juice, olive oil, kosher salt and fresh ground black pepper. (*Note: The radish salsa can become watery as it sits, so it’s best served about 10 minutes after preparation. Use a slotted spoon when serving the salsa to strain out any additional liquid.)
For the black beans:Mince the garlic. In a medium pot, heat 2 tablespoons olive oil. Add garlic and sauté for 1 to 2 minutes until the garlic is fragrant and barely starting to brown. Add the black beans along with the can liquid (add 1/4 cup water if using beans without liquid), several sprigs of cilantro, and 1/4 teaspoon kosher salt. Bring to a boil, then simmer 15 minutes until the liquid condenses.
Char the tortillas: If desired, char the tortillas by placing them on an open gas flame on medium for a few seconds per side, flipping with tongs, until they are slightly blackened and warm. (See How to Warm Tortillas.)
To serve, place the beans in the middle of a tortilla, top with radish salsa, feta cheese crumbles, and cilantro.