Preheat a grill to medium-high heat (375 to 450 degrees Fahrenheit).
Cut the bok choy in half. In a bowl, whisk together the olive oil, soy sauce, miso paste and kosher salt. Add the bok choy and toss until fully coated.
Place the bok choy directly on the grill grates and grill it for 3 to 4 minutes per side (6 to 8 minutes total), until tender.